Web orders as estimates?

We are currently in the middle of implementing NS into our business and have come accross a challenge we can't figure out.

On our old site orders come in sort of like estimates. People place an order, with no freight charges - and we do not ask for payment or credit card info. It's the first stage of our order process. Basically - the user adds item(s) to a cart, registers, then gets a "Thank you" page that basically tells them we're reviewing the order, confirming availability, and calculating freight.

Within a couple of days we'll "Finalize" the order in our system, which is sort of like turning it into an invoice but it doesn't actually invoice (key difference when talking to the accountant). Once finalized the system sends the client a link to login and pay for the order. Once they do it is invoiced.