One thing I've noticed is if you add an item to your basket then you shown the basket with the option to go to the checkout. If you do anything else, such as look at other t-shirts or read the t&c..you lose the ability to get back to the basket and checkout without trying to add something else to the basket..
Might be an idea to have the cart and checkout links included in a shop header on all shop-related pages..should be quite simple too (I've spent the past 2 weeks modifying shop code for a client and still have two weeks of php crunching left to go!)