Two suggestions:
use the “BUY NOW” button, which will allow you to use a dropdown menu that works on WordPress. (I tried it and it worked).
ALSO
I found this code on a WordPress.com forum
https://en.forums.wordpress.com/topic/paypal-dropdown-menu
which creates a drop down menu but I have no idea how to customize the code for my purposes. I called Paypal and they suggested I call a Paypal Developer. (not that serious)
https://www.x.com/developers/paypal
The code is below this line: Good luck!!
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[ Moderator Note: Please post code or markup snippets between backticks or use the code button. ]
<form target="paypal" action="https://www.paypal.com/cgi-bin/webscr" method="post">
<input type="hidden" name="cmd" value="_s-xclick">
<input type="hidden" name="hosted_button_id" value="ELGDYHSG6ERKQ">
<table>
<tr><td><input type="hidden" name="on0" value="Sizes">Sizes</td></tr><tr><td><select name="os0">
<option value="Small">Small </option>
<option value="Medium">Medium </option>
<option value="Large">Large </option>
<option value="Extra Large">Extra Large </option>
</select> </td></tr>
</table>
<input type="image" src="https://www.paypalobjects.com/en_US/i/btn/btn_cart_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>