I get this message:
We were unable to process your credit card payment. Please verify your card details and try again. If the problem persists, contact us to complete your order.I've seen related problems brought up in much older threads, like this one, though I haven't found a resolution.
I'm using Paypal Sandbox accounts and Paypal Web Payments Standard works OK. I've disabled the Shipping Quote block because all items have free shipping included. Using Test Credit Card number 4111111111111111 with CVV 123.
Once, I did get an Order Complete message in addition to the above error message. The payment did not go through to the Merchant account though.
Thanks in advance for any suggestions.

