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> *) allows the user to explicitly set CUSTREF. I did not know a good
> BOP field to use for this, so it needs to be used directly if the
> user needs it.

Looking at the Payflow Pro documentation, they suggest using the invoice
number, or a unique transaction identifier.

I could see doing one of two things:

1) Use invoice_number. We don't map this to anything else except
COMMENT2 right now, and it is what they suggest. Setting this to our
own unique transaction identifier probably isn't necessary to find
transactions, since we return the PNREF as order_number.

2) Define a new B:OP content field for unique identifiers and pass this
as CUSTREF (generate one if not passed?). We could use request_id?
(the current docs don't seem to mention anything about requiring it for
X-VPS-Request-ID or any X-VPS-* headers, actually. I wonder if they're
still necessary).

Leaning toward the first option.

-- 
_ivan