I've already replied to you via email, but we were unable to duplicate any problem with that product or the buttons on that page.
It sounds like you might have pasted the button code before customizing the product's Item Number in your Seller Admin, so the original number issued with the button code would would no longer work after the number gets changed. If that was the case, it appears you've fixed it now.
If you were testing with adding/deleting products in Admin at the same time as adding/removing items from a buyer cart on the same computer, that can sometimes confuse the cookie we set to track cart contents in your browser. Deleting your browser cookies should solve that problem for you.