I’ve recently investigated and fix the above error in Paypal for WooCommerce store. Here is more info about my findings and how I fixed it:
How the Issue Occurs
The message of the error is:
This invoice has already been paid. For more information please contact the merchant.
It means that for that Paypal account there is already such invoice which is already paid.
It may sound odd if in current WooCommerce store, there is order with same invoice number which is not paid. Customer would complain that he/she can’t complete the purchase.
The root cause of the issue is that for that Paypal account there is already such payment with such invoice which is already paid … which happens when the same Paypal account is mutual for different WooCommerce stores. So, here how the issue occurs:
A merchant has at least 2 different WooCommerce stores linked to the same Paypal account:
In store A, a customer completes an order with invoice ID = ‘WC-100’
In store B, another customer add product(s) to the basket and tries to complete the order with invoice ID = ‘WC-100’ (bad luck of invoice number generation).
Store B redirects the customer to Paypal and customer receive the above error, because the invoice ‘WC-100’ has already been paid.
The Solution
The solution is very simple and it is well described in WooCommerce settings:
1. Login to your WordPress Admin Panel
2. Navigate to Left Menu -> Appearance -> WooCommerce -> Settings -> Payments -> Paypal Standard (or actual Paypal payment method you use)
3. Change the ‘Invoice prefix’ to a unique value
Please Note: If you hover the tooltip for invoice the message is very helpful:
Please enter a prefix for your invoice numbers. If you use your PayPal account for multiple stores ensure this prefix is unique as PayPal will not allow orders with the same invoice number.
4. Save changes