credit cards via Empath: calculations done too late

["Alan Curry" <pacman-mvend@cqc.com>]

I'm trying to get a minivend catalog, based on the simple demo, to interface
with the credit card verification services of Empath Systems (www.empath.net)

The way they work is that our site (the minivend store) should, at checkout
time, send back a form whose ACTION attribute is a particular URL at
secure.empath.net. This form must include all the fields that are necessary
for credit card processing: firstname, lastname, ccnum, amount to charge, etc.
There is an additional hidden field called "returnto" which contains any URL
I want to put into it. The handler at empath returns an HTTP Redirect to this
returnto page after it records the transaction. (The actual credit card
verification is not done immediately; they are queued up and sent off in a
group once a day.)

My first idea was to just alter pages/ord/checkout.html to point to empath's
verification page. There are problems with that though. The order number
doesn't exist yet. The shipping destination may be altered, requiring the
amount of the charge to be changed. But this checkout.html page is the _last_
page before the final "receipt" is sent back to the customer, so I can't
find a good place to insert the hook to the empath server.

Has anyone done a minivend->empath link before, or anything similar?