You can manually tweak the "item_name" variable to create a drop down selection. Note, the online button creator is a great tool but, has it's limitations when you need some customization, thus the code needs to be manually created. For learning purposes, check out the example below:
<!DOCTYPE html>
<html lang="en">
<head>
<title>Basic Donation Example</title>
<!-- START META TAG SECTION -->
<meta name="Description" content="">
<meta name="KeyWords" content="">
<meta charset="UTF-8">
<!-- END META TAG SECTION -->
</head>
<body>
<!-- Start of Form -->
<form target="_self" action="https://www.paypal.com/cgi-bin/webscr" method="post">
<input type="hidden" name="cmd" value="_donations">
<!-- Replace "business" value with your PayPal Email Address or your Merchant Account ID -->
<input type="hidden" name="business" value="your PayPal Email Address or your Merchant Account ID">
<input type="hidden" name="no_shipping" value="1">
Choose Your Organization:
<select name="item_name">
<option value="Good Will" selected>Good Will</option>
<option value="United Way">United Way</option>
<option value="Boys and Girls Club">Boys and Girls Club</option>
<option value="West Side Shelter">West Side Shelter</option>
<option value="Pets and Vets">Pets and Vets</option>
</select>
<br><br>
<input type="hidden" name="no_note" value="1">
<input type="hidden" name="currency_code" value="USD">
<!-- Start Tax Override meaning No Tax -->
<input type="hidden" name="tax" value="0">
<!-- End Tax Override -->
<!-- <input type="hidden" name="bn" value="PP-DonationsBF"> -->
<input type="image" src="https://www.paypal.com/en_US/i/btn/x-click-but21.gif" border="0" name="submit" alt="Make payments with PayPal - it's fast, free and secure!">
<img alt="" border="0" src="https://www.paypal.com/en_US/i/scr/pixel.gif" width="1" height="1">
</form>
<!-- End of Form -->
</body>
</html>
