Here's a little tip for anyone using Paypal's Website Payments Standard system for electronic payments on their shopping cart. Their API allows you to pass in a complete shopping cart using a series of form fields listing the items, quantities, etc, as you'd expect. There's one tiny little detail that isn't explicitly stated - the button that you use to submit to the API must not have a name attribute otherwise it will fail. Here's a fully working example to better explain:
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As you can see there's no name attribute on the submit button, just an id.