Merchandise purchase is a well-worn path that web users are pretty familiar with, so I'll just skim the high points here.
Everything You Need To Know
The Minnesota book is probably the main product sold, so I'll use it in the illustration.
When you click on the "buy" button...
...you get to your shopping cart, showing this and any other items you've chosen. You can alter the number of any item purchased, or clear the cart entirely.
We hope that eventually you'll push the "checkout" button...
Which shows you the order, with a subtotal, including sales tax and shipping cost. This is a fairly simple-minded implementation. We currently sell three pieces of physical merchandise off the site, and the weights don't vary too much, so we have a shipping price associated with each product and leave it at that. No savings combining multiple products in an order, etc. And that sales tax amount goes away on the next page if the address you enter isn't in Minnesota. This makes the checkout process one screen shorter than it would be otherwise. And probably wasn't worth it, though there haven't been a lot of complaints from confused customers.
When you continue...
Shipping and Billing
...you get to this page where you enter the shipping and billing information. We don't have provision for shipping to a different address, and we don't currently take advantage of the extra security code on the back of credit cards.
If you get an error on this page, you get this same page back, with the error indicated and all your information present so you can correct it and resubmit.
When you successfully enter shipping and billing information, you get a final order confirmation page (not shown; it's dull). The one interesting point is that the page gives you a link that you can use in future to check the status of the order—whether we think we've shipped it yet.