I'd like to be able to choose the color and opacity of the overlay layer that is created during the Checkout process. I've had multiple people tell me that they get stuck because they don't notice the checkout "Save Choices" button that takes you past the first page of the checkout. They click and click and then give up. I think either the bar at the bottom needs to have higher contrast so it doesn't get lost, or the overlay should be adjustable to so both modules that appear are highlighted better.
tl;dr - the checkout popup's white on white divs is problematic and keeps the potential customer from knowing what to do next).