Fatal error: Call to undefined function get_display_price() during checkout

I am using Stripe in test mode and I have not configured any emails in settings. The email address used in checkout was my Paypal sandbox test buyer (I was trying to get that to work and couldn't so I am trying Stripe) so I don't know if it's a valid email.

Upon confirm-checkout, I get

Fatal error: Call to undefined function get_display_price() in /home/caitlindean/tinymachineconsulting.com/dev_tangora/wp-content/plugins/marketpress/marketpress.php on line 4164

This occurs in the filter_email() function (screens included below)

I've got a week's worth of time invested in this plugin, so I really need to make it work for this site. I have a few open issues here, but this is a showstopper.