Order processing interface

[Steve Baker <steveb@web.co.nz>]

I have just installed MiniVend/MiniMate to evaluate it for possible
future shops.  I think I have a reasonably good feel for what it can do.
The gaps in my understanding relate to the interface that a Merchant
would use to process orders.

I will have to write an interface to a Credit Card authorisation
provider here in NZ.  I think it would be fairly easy to configure the
site to debit the credit card when the user submits their order.
However, I would need something which does the following:

-Authorise the card at the time of order
-Allow the merchant to debit accounts as they manually process orders
(through the admin pages)
-Give the merchant an interface to manage partial-order-fulfillment,
refunds, tweaking shipping and other costs for each order in the
database.

Basically the type of stuff that OpenMarket (buggy bloated ugly)
Transact does.

I wouldn't mind writing this kind of interface myself, but I would like
some advice on the best way to do it.