First, bear in mind that if you change the Minimum or Suggested Price setting for an item that lets buyers edit the price, you would need to recopy fresh button code to paste into your site. This is because the Sugg. Price simply adds an &amount= parameter to the button URL, which adds the specified amount to the item's base (Min.) price, so changing the difference between the Min. vs. Sugg. Price settings would change the &amount= value in the button code.
Also bear in mind these tips for testing your shopping cart:
http://www.e-junkie.com/ej/help.test.htm
I tested the button URL for your most recently-edited product but was unable to reproduce the problem you described; the &amount= and &custom= parameters worked as expected for me. If you haven't already sorted the issue for yourself by now, we'd need to know which of your products is affected and see the problem happening in a live Web page. If you'd rather not post that information here, you can email it to support:
https://www.e-junkie.com/ej/contact.htm
