I think the site is very nice - nice, clean and simple. My only suggestions are personal preferences really, and I'm sure others, yourself included, may disagree.
I'm not sure if the images might look better with a slight border around them - perhaps a little padding, then a 1-pixel border. There just seems to be something missing there.
I'm not a fan of the button-like push-down effect that you've added to everything, but mainly the images. At first I thought it was a bug until I noticed it was on all the text links too. For me it kind of cheapens the site.
I don't like the orange hover colour on the buttons (and orange is even my favourite colour!) - would it not look better if you stuck with the deep red colour you've used on all links?
I think there should be a more white-space between the main page and your footer. Once I get down to the bottom of the page, my focus is drawn to the footer because it contrasts so much with the main page - I think my focus should remain on the product.
The header/logo... there's just something needed here to really finish everything off.. but I'm not sure what. At first I thought the header would look better if it was set within the black bar at the top (the bar being extended down of course) but then you'd need to play about with the navigation links on the right and I think they look fine as they are.
I'm not sure if it's been used so much on the web now that it's a bit tiresome, but perhaps just giving the header text a letter-press effect..?
On the help page, you need to add some padding to the top of the text box - perhaps the same amount that is on the left of the box.
I expected the Powered by Paypal text to be a link because everything else in the footer is a link and the text looks identical. Not a massive deal, but it might be nice to either make it link to something or change the style slightly so it's obvious it's not clickable.
As I said, these are all just personal preferences - you should be proud and happy to go with the site as it is now