Forum Moderators: buckworks
If you mean the invoice - I'd guess yes from my own experience. When I print the invoice for my records I only want the relavent info and not all of the additional trailing info like ads.
For retailers selling non-expensive type product, I'd agree with the above posters. Get them out (1 Form) as soon as possible before they change their mind. On most of our sites, we offer the ability to register, but many times do not require it. Suprisingly, a non-trivial amount of people chose not to register and just to checkout.
For other types of sites, it may make logical sense to do it other ways. For example, on sites we design that are selling event registrations, it's often necessary to design a two or three stage form that handles number of registrants, registrant information, then payment information. This is because the purchaser is not always the attendee, and usually with events (or other high-priced product), the decision won't be reversed by an intimidating form.
Tip: Consider putting your phone number at the top of your checkout form. Fax information, too. You'll find that people will actually call you... sometimes I do wonder if sales would be lost without that.