Problem Finalizing Orders - Marketpress

We have been using Marketpress on our site at http://foodnbeverage.org for over a year with Authorize.net emulation. We never had a problem, but then mid December our customer would get the following error when when confirming payment
There was a problem finalizing your purchase. sans-serif;background:#EEEEEE;} fieldset{padding:0 15px 10px 15px;} h1{font-size:2.4em;margin:0;color:#FFF;} h2{font-size:1.7em;margin:0;color:#CC0000;} h3{font-size:1.2em;margin:10px 0 0 0;color:#000000;} #header{width:96%;margin:0 0 0 0;padding:6px 2% 6px 2%;font-family:"trebuchet MS" Please go back and try again.

I think it happened right after we upgraded WordPress - not really sure, but it happened without anyone messing with it from the admin panel. I thought / hoped it was because the plugin needed to be upgraded to work with the latest WordPress, so I just purchased it and still get the error. Any help or insight into this would be appreciated more than you know.

Thank you,
Chuck