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My fear is that it will be a nightmare to match the data gathered from the in-house form with the data from the outside vendor. (7,000 - 10,000 attendees will register on-line)
You take the incoming data and use curl or LWP to post it to the payment processor. The payment processor will have an API which specifies certain data field names. Usually they are pretty self-explanatory as to what type of data they are looking for. Most processers publish good documentation for their APIs.
When building the POST string, you simply assign the incoming data with the appropriate API field names, and wala, you have matched data. It really isn't difficult.