Checkout: Change the Orders Database Index
Chances are that you might be getting an “Order has already been processed”, “Invoice has already been paid for” or “The order already exists in the database” message.
If you have used your payment gateway/method with other shopping cart software in the past, it is possible that invoice IDs have been logged at your payment gateway. Since this plugin starts your orders at 1 again and increments with each new order, your payment gateway might be thinking that it is trying to process old invoices.
The solution is to change the database index on the Orders database table named “wp_wpcoorders” (considering that your prefix is ‘wp_’). Here is a video which illustrates how to do it: