Some small comments:
* When I click on "checkout", a few things are not clear: how much is shipping (seems to be free?), how long will shipping take, etc.
* I put something with an "&" in the title into my shopping cart. On the PayPal landing page, it turns into &. Also, the item description is "shopid: x, itemid: y", which is not very helpful, especially if it's two months from now and I'm trying to remember what I purchased.
* The list of item in the shopping cart should be links, not text.
* It's an MVP, so I'm sure this is on your "next steps" list, but it would be nice to delete specific items from the cart instead of having to clear the whole cart.
* There's a Login/Signup link at the top, but it's not clear what the benefits of Signup would be. Clicking on the link doesn't reveal the benefits either.
Again, this is awesome.