PayPal's Payments Standard does not have any built in features or options on the PayPal Checkout Screens for discounts or coupons. What is available are the HTML "discount" variables. Thru the use of these variables and some custom scripting, you can create either a mini-cart or item button that allows your customers to insert a promotional code or coupon for a discount. It's not too difficult if you're into manually coding your own item buttons as what have in mind cannot be done using the online button creator.
If you're not in to coding then you would need to find and use a 3rd party shopping cart that has the features you need. For example, check out this one, as it's easy to work with:
http://www.e-junkie.com/
To learn more about the various "discount" variables, you can find info in this desktop guide:
https://www.paypalobjects.com/webstatic/en_US/developer/docs/pdf/pp_websitepaymentsstandard_integrat...
If you like to code, this example should provide you with some idea of what can be done using the discount variables and some scripting. You can even change the script to a .js file which keeps it separate from the HTML file.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<title>Button Example Code</title>
<!-- START META TAG SECTION -->
<meta charset="utf-8">
<meta http-equiv="Content-Type" content="text/html;charset=utf-8">
<meta http-equiv="Content-Language" content="en">
<!-- END META TAG SECTION -->
<!-- Start of Script -->
<script language = "javascript">
<!--
function validate(text1,text2) {
if (text1 == text2)
{
window.alert("Nice, you get a 10% Discount!");
}
if (text1 !== text2)
{
window.alert("Sorry, No Discount!");
}
}
function CalculateOrder(form) {
if (form.text1.value == "GOLD10")
{
form.discount_rate.value = "10";
form.discount_rate2.value = "10";
form.on3.value = "Coupon Entered";
form.os3.value = "GOLD10";
}
}
//-->
</script>
<!-- End of Script -->
</head>
<body>
<!-- START SAMPLE CODE SECTION -->
<!-- PARAGRAPH 1 -->
Discount & Coupon Examples
<br><br>
Add to Cart - Enter the Promo Coupon Code, Get a 10% Discount, on Screen Validation.
<br><br>
Notes:<br>
This example uses a script to verify the Promo Coupon Code and then applies the associated Discount Rate using the the "discount_rate" and the "discount_rate2" variables.
<br><br>
Give it a try, the Promo Coupon Code is: GOLD10
<br><br>
<!-- START CONTENTS -->
<!-- START BUTTON EXAMPLES -->
Widgets in Space
<br><br>
Widget Cupcakes
<br><br>
<!-- Start of Add to Cart Form -->
<!-- Note: target="paypal" was replaced with the variable target="_self" -->
<!-- Note: shopping_url also added to code -->
<!-- These two changes allow better functionality with IE and Firefox -->
<form target="_self" action="https://www.paypal.com/cgi-bin/webscr" method="post">
<!-- If using a Business or Company Logo Graphic, include the "cpp_header_image" variable in your View Cart code. -->
<input type="hidden" name="cpp_header_image" value="https://yourwebsite.com/logo.jpg">
Please Choose:  
<input type="hidden" name="on0" value="Choice">
<select name="os0">
<option value="Standard, 6 each per Box" selected>Standard, 6 each per Box - $9.95</option>
<option value="Standard, 12 each per Box">Standard, 12 each per Box - $16.95</option>
<option value="Deluxe, 6 each per Box">Deluxe, 6 each per Box - $12.95</option>
<option value="Deluxe, 12 each per Box">Deluxe, 12 each per Box - $22.95</option>
</select>
<!-- option_index value=0 is associated with variable on0 -->
<!-- option_index value=1 is associated with variable on1 -->
<input type="hidden" name="option_index" value="0">
<!-- -->
<input type="hidden" name="option_select0" value="Standard, 6 each per Box">
<input type="hidden" name="option_amount0" value="9.95">
<input type="hidden" name="option_item_number0" value="S-6-BOX">
<!-- -->
<input type="hidden" name="option_select1" value="Standard, 12 each per Box">
<input type="hidden" name="option_amount1" value="16.95">
<input type="hidden" name="option_item_number1" value="S-12-BOX">
<!-- -->
<input type="hidden" name="option_select2" value="Deluxe, 6 each per Box">
<input type="hidden" name="option_amount2" value="12.95">
<input type="hidden" name="option_item_number2" value="D-6-BOX">
<!-- -->
<input type="hidden" name="option_select3" value="Deluxe, 12 each per Box">
<input type="hidden" name="option_amount3" value="22.95">
<input type="hidden" name="option_item_number3" value="D-12-BOX">
<!-- -->
<br><br>
Cupcake Flavors:  
<input type="hidden" name="on1" value="Flavor">
<select name="os1">
<option value="Vanilla Dream" selected>Vanilla Dream</option>
<option value="Berry Burst">Berry Burst</option>
<option value="Chocolicious">Chocolicious</option>
<option value="Carrot Passion">Carrot Passion</option>
<option value="Banana Karma">Banana Karma</option>
<option value="Mocha-Chococcino">Mocha-Chococcino</option>
</select>
<br><br>
Optional Gift Message:  
<input type="hidden" name="on2" value="Optional Gift Message">
<input type="text" name="os2" maxlength="60">
<br><br>
<!-- Start Coupon Validate -->
Enter Promo Coupon - Get 10% Off:
   
<input type="text" name="text1">
<input type="button" value="Check It" name="Submit" onclick=javascript:validate(text1.value,"GOLD10") >
<!-- End Coupon Validate -->
<br><br><br>
<input type="image" src="https://www.paypalobjects.com/en_US/i/btn/btn_cart_LG.gif" border="0" onclick=CalculateOrder(this.form) name="submit" alt="Make payments with PayPal - it's fast, free and secure!">
<img alt="" border="0" src="https://www.paypalobjects.com/en_US/i/scr/pixel.gif" width="1" height="1">
<input type="hidden" name="add" value="1">
<input type="hidden" name="cmd" value="_cart">
<!-- Replace "business" value with your PayPal Email Address or Account ID -->
<input type="hidden" name="business" value="your email address">
<input type="hidden" name="item_name" value="Widget Cupcakes">
<input type="hidden" name="no_shipping" value="2">
<input type="hidden" name="discount_rate">
<input type="hidden" name="discount_rate2">
<input type="hidden" name="on3">
<input type="hidden" name="os3">
<!-- Replace value with the web page you want the customer to return to -->
<input type="hidden" name="shopping_url" value="http://www.yourwebsite.com/Shop.html">
<!-- Replace value with the web page you want the customer to return to after a successful transaction -->
<input type="hidden" name="return" value="http://www.yourwebsite.com/ThankYou.html">
<!-- Replace value with the web page you want the customer to return to after item cancellation -->
<input type="hidden" name="cancel_return" value="http://www.yourwebsite.com/Cancel.html">
<input type="hidden" name="currency_code" value="USD">
<input type="hidden" name="lc" value="US">
<input type="hidden" name="button_subtype" value="products">
<input type="hidden" name="no_note" value="0">
<input type="hidden" name="cn" value="Add special instructions to the seller:">
<input type="hidden" name="bn" value="PP-ShopCartBF:btn_cart_LG.gif:NonHosted">
</form>
<!-- End Add to Cart Button Code -->
<br>
<!-- Start of View Cart Button Code -->
<form target="_self" action="https://www.paypal.com/cgi-bin/webscr" method="post">
<!-- If using a Business or Company Logo Graphic, include the "cpp_header_image" variable in your View Cart code. -->
<input type="hidden" name="cpp_header_image" value="https://yourwebsite.com/logo.jpg">
<input type="hidden" name="cmd" value="_cart">
<input type="hidden" name="display" value="1">
<!-- Replace "business" value with your PayPal Email Address or Account ID -->
<input type="hidden" name="business" value="your email address">
<!-- Replace value with the web page you want the customer to return to -->
<input type="hidden" name="shopping_url" value="http://www.yourwebsite.com/Shop.html">
<input type="image" src="https://www.paypalobjects.com/en_US/i/btn/btn_viewcart_LG.gif" border="0" name="submit" alt="PayPal - The safer, easier way to pay online!">
<img alt="" border="0" src="https://www.paypalobjects.com/en_US/i/scr/pixel.gif" width="1" height="1">
</form>
<!-- End of View Cart Button Code -->
<!-- END BUTTON EXAMPLES -->
<!-- END SAMPLE CODE SECTION -->
<br><br>
<br><br><br><br>
<hr align="left" width="50%" noshade>
<br><br>
NOTES:
<br>
In order to test the code, you must replace the "business" value variable with your PayPal Email Address or Account ID in both the View Cart button and the Add to Cart button.
</body>
</html>
