The following was tested on Internet Explorer, FireFox, and Opera: worked on all three.
Code:
<form action="https://www.paypal.com/cgi-bin/webscr" method="post">
<input type="hidden" name="cmd" value="_xclick" />
<input type="hidden" name="business" value="$post[field5]" />
<input type="hidden" name="page_style" value="PayPal" />
<input type="hidden" name="no_shipping" value="1" />
<input type="hidden" name="no_note" value="1" />
<input type="hidden" name="tax" value="0" />
<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>
The only difference I see is that I removed the currency and I changed shipping from 2 to 1.