Authorize.net Batch transactions / Mass Payment - 'how to'

[eluser]phpmonster[/eluser]
I will be building a micro scale site of groupon using CI.
The client wants to use Authorize.NET as their payment gateway.

Has anyone build a mass payment library for Authorize.net using CI, can someone point me in the right direction.

Typical scenerio would be: pre-authorize cards until quantity hits lets say 50 sales.
Once 50 sales are hit, send 50 transactions to Authorize.net for mass/ batch processing.

I imagine I need to store all pre-authorizations in a db table, then once it hits x amount, grab those and send them to Auth.net. Direction on the steps would be greatly appreciated.