Payment Information page is blank

Hi...when I try to buy one of the sample items from our site's store at hypnomaps.com/store, the checkout page appears blank. When I click 'Continue,' I get the following message:

There was a problem connecting to PayPal to setup your purchase. Please try again.
10400 - Transaction refused because of an invalid argument. See additional error messages for details. - Order total is missing.

I'm assuming this is some setting that I missed, but I can't find anything.