Marketpress - Paypal Express Checkout Does Not Work

Hello all,

I've been on the phone with Paypal for 3 hours now, and we are both looking at Marketpress, trying to figure out why the following issue is occurring. I'm posting here because I desperately need help from the WPMU gurus.

I'm using Marketpress w/ WPtouch to set up a mobile store. Marketpress does not work w/ Paypal Mobile Express Checkout out-of-the-box. After he reviewed the Paypal Developer docs, Aaron suggested that all I had to do was add the cmd=_express-checkout-mobile argument to the redirect url.

That worked. Orders now go straight to Paypal's Mobile Express Checkout where people can enter their credit/debit card information, BUT....

I've tested 5 different credit/debit cards, and none of them are accepted by PayPal. Instead, this error appears:

"The card you entered cannot be used for this payment. Please enter a different credit or debit card number."

A Paypal developer and myself have been trying to figure out if this is a PayPal issue or a Marketpress issue. But, the Paypal developer cannot see the Request that Marketpress sends to Paypal so that he can look up the information in Paypal's error log.

Has anyone experienced this error before? A google search reveals this error message to be pretty pervasive. Does anyone know if this is a result of information that Marketpress is sending over?