Invoice Number or Payment Number in Business::OnlinePayment transaction?

[Nicholas Moline]

I don't know if you guys will consider this a silly question or not, but 
my merchant account charges me a higher rate on transactions if I do not 
pass an order or invoice number along with the transaction when 
processing the credit card.  I've been trying to find the part of the 
code where it actually calls the Business::OnlinePayment transaction so 
that I can alter the call to pass one of those identifiers along with 
the transaction so I can get the lesser rate, but for some reason I've 
found myself at a loss in locating this bit of the code.  I hate to 
bother the list with a question that should be a trivial matter of 
finding a simple subroutine call, but hopefully this is a question that 
someone can answer quickly while being only minimially annoyed with me 
for asking.

Thank you in advance,

Nicholas Moline