I looked at a few of those and quickly got disoriented.
The solution I opted for was to set up a Google Checkout account for the client. Then I created one "Add to Cart" button for registration and one "Add to Cart" button for the optional t-shirt. Then I wrote a few instructions of what to do and I'm done.
I think it will accomplish what is needed. We'll see.
NOTE: I originally set all this up using PayPal, BUT when I saw that they doubled their fees I switched to Google Checkout. Bad, PayPal! Bad!