If you receive an error message using Paypal that reads:
“This transaction is invalid. Please return to the recipient’s website to complete your transaction using their regular checkout flow.”
you have PayPal’s Express Checkout turned on, but you are not using PayPal Direct Payment. The Express Checkout feature can be turned on in the Cart32 administration under “Cart Settings” and “Paypal” tab, but it doesn’t prompt you to change your payment processor (under the Enterprise Tab) to Paypal Direct Payment. The Express Checkout requires this Paypal Direct Payment and will cause an error on ALL PayPal transactions for your client by giving the above error message if PayPal Direct Payment is not turned on.
If you are using a payment processor other than PayPal Payments Pro, you will have to turn off the Express Checkout feature or NONE of the paypal methods will work.
If you are using PayPal Direct Payments, follow the steps necessary to set it up under the Shopping Cart tab of the Cart32 administration for the Express Checkout feature to work properly.