Stripe Error: “Error: Failed to process the order, please try again later. Message: No such customer: cus_XXXXXXXXXXXXX; a similar object exists in live mode, but a test mode key was used to make this request.”

Usually it happens because you are trying to purchase something in Live mode, using the same billing email you’ve used for testing the checkout process, when the Test mode was enabled. 

In One Click Upsells plugin customer’s information is linked to his email, so that he can checkout despite the fact he is logged in or not.

Solution: Try using different emails, when you are testing the checkout process in test and live modes.