Problem testing Paypal Sandbox

So, I'm testing my site to enable sales of digital downloads.

I've created a test product and uploaded the downloadable file through the product edit page. The product is published.

I've created a Paypal sandbox user and a Paypal sandbox biz account. I've entered the biz credentials into my store details for payment using only Paypal Express checkout and making sure that the sandbox is enabled.

However, once I try to test the checkout process, I get the following symptoms.

I am allowed to add the product to my cart. The Items in Cart does not get updated and still reflects 0 items. However, when I unHide the cart, the product is there, and I am able to proceed to checkout. When I click the checkout button, then I get the "no items in cart" page instead of the rest of the process.

Am I running the sandbox wrong? Is there something wrong with my product data? My sandbox accounts are enabled.

That my cart indicator is not updated is very fishy to me. Am I missing a switch somewhere?

Thanks for your help!