use dynamic payment amount with Amazon Payments

Remove signature from POST attributes because we're dynamically
adjusting the content. This would potentially be a problem, but because
the price is completely user-defined, we're not impacted by a developer
attempting to adjust that value during purchase.
match payment amount with display on page load
temporarily hide PayPal option

we need to create server-side calls to build these requests to send and
receive information from PayPal