Googlecheckout allows you to CURL the customers order, they return a URL and then you send the user to the URL, so they can not see the data you sent good and thus they can not modify it.
I see there is polling requests for Amazon Checkout with CURL, but is it does the same function exist in Amazon Checkout, or Amazon FPS (flexible Payment system)?
Google Code: https://developers.google.com/checkout/developer/Google_Checkout_HTML_API#api_details https://developers.google.com/checkout/developer/Google_Checkout_HTML_API#integration_overview // CURL Example, scroll a down little bit from here.
_type=checkout-shopping-cart
&shopping-cart.items.item-1.item-name=Peanut%20Butter
&shopping-cart.items.item-2.item-name=Strawberry%20Jelly
&shopping-cart.items.item-1.item-description=Made%20from%20peanuts
&shopping-cart.items.item-2.item-description=Made%20from%20strawberries
Thus the request would look like this.
curl -d '_type=checkout-shopping-cart&shopping-cart.items.item-1.item-name=Peanut%20Butter&shopping-cart.items.item-2.item-name=Strawberry%20Jelly&shopping-cart.items.item-1.item-description=Made%20from%20peanuts&shopping-cart.items.item-2.item-description=Made%20from%20strawberries'
https://P_MERCHANT_ID:[email protected]/api/checkout/v2/requestForm/Merchant/P_MERCHANT_ID
Sadly amazon is lacking in documentation or it is at least hard to find. You might try asking them at https://aws.amazon.com/support/