What you have in mind is doable, it's just requires a different approach. Check out the example below, it should provide you with a few ideas, feel free to tweak.
<!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 -->
</head>
<body>
<!-- START SAMPLE CODE SECTION -->
<!-- PARAGRAPH 1 -->
Donation Examples
<br><br>
Make a One Time Donation or setup a Donation Schedule like a Subscription.
<br><br>
<!-- Start of Form -->
<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="_xclick-subscriptions">
<!-- <input type="hidden" name="redirect_cmd" value="_xclick-subscriptions"> -->
<!-- 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 Fund Donation">
<input type="hidden" name="item_number" value="WFD-1002">
<input type="hidden" name="no_note" value="1">
<!-- no_shipping value 2 needed to send the free gift -->
<input type="hidden" name="no_shipping" value="2">
<input type="hidden" name="tax" value="0">
<input type="hidden" name="lc" value="US">
<input type="hidden" name="currency_code" value="USD">
<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-BuyNowBF:btn_buynow_LG.gif:NonHosted">
<!-- 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">
<!-- -->
Enter the amount you would like to donate:
<!-- a3, Required, Regular Subscription Price -->
<input type="text" name="a3" size="4" maxlength="7">
<br><br>
<!-- src, Optional, Recurring Payments, 0 no recur, 1 payments recur -->
<input type="hidden" name="src" value="1">
<!-- -->
Select the length of the desired Billing Cycle:
<!-- p3, Required, Duration based on the value of t3 -->
<select name="p3">
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
<option value="5">5</option>
<option value="6">6</option>
<option value="7">7</option>
<option value="15">15</option>
<option value="30">30</option>
</select>
<br><br>
<!-- -->
Select how often you would like to Donate:
<!-- t3, Required, Subscription Units D W M Y, -->
<select name="t3">
<option value="D">Day(s)</option>
<option value="W">Week(s)</option>
<option value="M">Month(s)</option>
<option value="Y">Year(s)</option>
</select>
<br><br>
<!-- -->
Select the number of Donation Payments you would like to make:
<br><br>
<!-- srt, Optional, Recurring Times, must be used with src -->
<select name="srt">
<option value="">Until Canceled</option>
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="6">6</option>
<option value="12">12</option>
<option value="18">18</option>
<option value="24">24</option>
<option value="36">36</option>
</select>
<!-- sra, Optional, Reattempt on failure 2 more time, value 0 or 1, if 0 no reattempt -->
<input type="hidden" name="sra" value="1">
<br><br><br>
Thank you for your Donation,
<br><br>
Please Select your Free Gift:
<input type="hidden" name="on0" value="Free Gift Choice">
<select name="os0">
<option value="Solar Calculator">Solar Calculator</option>
<option value="Key Chain Light">Key Chain Light</option>
</select>
<br><br><br>
<!-- -->
<input type="hidden" name="bn" value="PP-SubscriptionsBF:btn_donateCC_LG.gif:NonHosted">
<input type="image" src="https://www.paypalobjects.com/en_US/i/btn/btn_donateCC_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 Form -->
<!-- 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.
</body>
</html>
