I was searching for a way to display the cart on the shipping page before being redirected to Paypal to make a payment and saw a post, but can no longer find it.
Anyway...after digging through files and lots of trial and error I found a way.
By editing the template-functions.php file I copied the cart function, named the function something else, and then called my modified function in mp_show_cart - voila - the total displays!
I attached the file so that anyone else can use it - it goes in wp-content/plugins/marketpress/marketpress-includes AND needs to have the .txt removed off of the filename.
Hopefully an updated version of MP will show the current cart total AND update the total as shipping type is selected, but the WAY cool method would be a fully customizable checkout flow.