Error Completing Transaction

Greetings…I have worked diligently for two weeks to develop a pretty nice looking Markpress site built on the Thesis 2.0 framework. Everything was going great through testing until the final step.

When hitting the Confirm Payment button on the last step of a live purchase:

When executing the final step of the order I get this error message:

Fatal error: Call to undefined function get_display_price() in /home/gsiww/public_html/wp-content/plugins/marketpress/marketpress.php on line 4164

This transaction does get completed, in that I can see the transaction charged in Pay Pal account and I see the orders appear in the Manage Orders area. But the error does not allow the user to be redirected to a reassuring thank you page etc.

Please advise as soon as possible.