The key here is that the button leading to 'register' was 'checkout', and not 'register'. Because web links are essentially blind corners, where users can't see around the corner, the process involves a certain amount of trust between the user and the guide. For example, imagine being at a fair ground, and without knowing where you were there's a sign that said 'Ferris Wheel' and following that sign took you the ticket counter instead. Sure you need a ticket to get on the ferris wheel, but you were looking for the ferris wheel and not the ticket counter. This is the same thing. Checkout should lead to checkout. Register should lead to register. Checkout shouldn't lead to register. Once you've checkedout, you can close the transaction by creating an account for future purchases, but that's only because you've already completed all the necessary work, and so creating an account becomes a value added bonus.

Simple directions, simple prompts, and mechanize the rest. (Zip Code? -- Fill in the City automatically.) as mentioned by jobu below