How does Bonanza.com do this?
You can purchase an item from a seller on bonanza.com with Amazon checkout and it will bring you back to bonanza.com after the transaction. All of this is done without requiring the seller to update their return URLS in their checkout pipeline settings.
Bonanza just requests the sellers merchants id, access key, and secret access key. How can I also do this without requesting users to add a return URLS in their settings?
Bonanza says they send all necessary information to Amazon during checkout, and do not need to set any “Checkout Pipeline Settings” in Seller Central. Any settings you have there will not be used for Bonanza transactions.
I'm not familiar with Bonanza.com but it sounds like they are using the Amazon Flexible Payments Service. With FPS you send your customers to a co-branded UI at Amazon to complete a purchase and after they've completed the purchase the customer is returned to a URL that you have specified as a query parameter in the link used to send the customer to Amazon.