Hello,
We recently switched from using the PayPal Express Checkout with IPN to the REST API and while almost everything appears to be working just fine we have one pretty serious issue. We have received complaints from customers that paying via PayPal on our website doesn't work for them while it works just fine with other e-commerce websites. Upon further investigation it seems that the problem occurs only when someone attempts to pay by card instead of PayPal balance, I have also verified that the error does not occur on sandbox but only on live.
The exact message I get from the PayPal API when this error occurs according to the server logs is "[INSTRUMENT_DECLINED] The instrument presented was either declined by the processor or bank, or it can't be used for this payment." with a 400 HTTP status code. When that happens it's always at the final stage when the payment is executed.
I know that the problem can't be with the bank declining the payment because I have also tested myself making a payment and paying by card from two different PayPal accounts and in both cases the issue occurred. And just after that I attempted making a purchase from a different website and everything worked just fine. Finally, if I make sure my PayPal balance isn't 0 and use that instead to make the payment then it always works without a problem.
