I guess I'm on an e-commerce kick (I even created a new category for it).
Today, I'd like to point you to this post from Get Elastic (an e-commerce blog; worth subscribing to) on how the very best e-Commerce sites handle checkout.
Lots of good information for you to use when looking at your own checkout process. You *have* actually bought something from your own site, right?
This also brings me to a pet peeve: inflexible e-commerce providers. When you are paying for a (hopefully integrated) e-commerce solution from a vendor, you need to be able to control what is going on in the checkout process. Once a customer decides to buy something, you want to make sure that you are doing everything possible to help them complete the transaction.
If your vendor or Webmaster doesn't agree with this, find another source. Losing customers because your e-commerce provider is inflexible is simply unacceptable.