We have just finished some custom PayPal integration for Eaton Square Concerts (ESC). On the ESC booking page, you can now select all the tickets you want before heading to PayPal, thanks to the PayPal “cart upload” feature. Previously, the site was using the single-item “Add to cart” which is a bit clunky, and means the user heads back and forth between the ESC site and PayPal.
We handle the form submission on the ESC site, work out what the user want to buy, read their response to the optional questions, and then send the form off to PayPal. The intermediate step is needed because PayPal refuses to accept a zero-count for items, forcing us to check the item count before POSTing to PayPal.