Opayo (formerly called 'Sagepay') is one of the most popular credit card payment providers in the UK.
VSP Form is a remote-type payment system, where customers are sent to a secure page on Opayo's servers to conduct the card transaction.
The system is simple to set up, but is unusual compared to most remote type payment systems in that the 'callback' (relaying information on a successful payment back to the store) is achieved by redirecting the user's browser back to the store. Most payment systems make the callback themselves (their server calls the store's callback page directly), even if they subsequently send the user back to the store after payment. In theory, payments might not be notified back to your store if a user closes their browser immediately after the payment was made, but in practice, we rarely see this and the system seems stable and reliable.
All the required information to process an order and direct the callback back to your store is actually posted through to Opayo with the order, so no further setup is required at their end. However, you should ensure with Opayo's support that your account is live before you start processing orders.
Orders passed through your Opayo account must have unique IDs. If you experience this error when trying to checkout, it means the order ID that is being passed has been used before on your account. This can happen if you've switched to Kartris from another store system, so IDs are starting from a low level. It can also happen if you restored a older backup of your Kartris database for some reason.
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