lloking at the source of your page, I see a number of badly formatted buttons.
for example, the first button on your page works as expected, because it has all of the pertinnant information:
<form 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="6MS8VEQCVVE7W" />
<img alt="" border="0" src="https://www.paypalobjects.com/en_US/i/scr/pixel.gif" width="1" height="1">
</form>
the other buttons on your page are missing the hosted_button_id element:
<form action="https://www.paypal.com/cgi-bin/webscr" method="post">
<p>
<input type="image" src="https://www.paypalobjects.com/en_US/i/btn/btn_buynow_LG.gif" border="0" name="submit" alt="PayPal - The safer, easier way to pay online!" /></p></form>