In the past I have opened the shopping cart in a new window, thus half fixing the problem. Truthfully this is not necessary though. You should be able to write your own form that links to the specific user's shopping cart.
Simply make a link to the PayPal shopping cart in your theme code or directly in your page or post code. Ideally this could be generated by a ShortCode or something but currently it is not. It also doesn't need to be generated by a ShortCode because it is pretty much a static form.
Perhaps I could register a special nav menu item that WordPress Admins could put in their navigation menus.
Personally I would recommend adding a "Shopping Cart" link or button in your top navigation area. I would also recommend doing some more research online on how to setup a PayPal shopping cart.
PayPal Button Making Documentation:
Code For a View Cart Button:
<form name="_xclick" target="paypal" action="https://www.paypal.com/cgi-bin/webscr" method="post">
<input type="hidden" name="cmd" value="_cart">
<input type="hidden" name="business" value="">
<input type="image" src="https://www.paypal.com/en_US/i/btn/view_cart.gif" border="0" name="submit" alt="Make payments with PayPal - it's fast, free and secure!">
<input type="hidden" name="display" value="1">
Keep in mind you may us text or other images for the shopping cart button, it is up to you and how you make the form. Good luck, Cheers