MarketPress Issue with checkout

Hello! I just launched the site, and have an issue that I can’t figure out. I did some test transactions, and this is what I have come up with. First, I am using Paypal Express.

When I am logged into my WordPress account, the transaction goes through perfectly (I checkout as guest). Then I log out of WordPress, and checkout as guest again, and the transaction does not work. I get an error:

“12114 – Transaction refused because of an invalid argument. See additional error messages for details. – Invalid item URL.”

I looked down the page and I get a message that says that the transaction is going to be processed through my Paypal account, even though I checked out as a guest, and put in an email that is not associated with a Paypal account.

The glitch is that even though I am checking out as a guest, Marketpress and Paypal are trying to process a Paypal account, and not the credit card info that I put in.

Please help.