SIM module problem: "This transaction cannot be accepted" | Ubercart:

I am trying to setup the this module (SIM) payment method... but I keep getting this error when I try to complete a transaction:

'The following errors have occurred.
(99) This transaction cannot be accepted.'
Where exactly do I place my MD5 hash or what may be creating this error?


And the right answer, whether you're using Ubercart or, in my case, rolling your own SIM cart...

The has[h] is an MD5 of your API login ID and a few other fields in the transaction. Have you entered the correct login ID?

Welp, that's it. Whatever it is that you're doing, it's the fingerprint that's screwing up. I'm allowing customers to change quantities on the order page, so of course we've got to allow the amount to change as well. That screws up your fingerprint, and you have to generate a new one for each amount. I'm doing that with AJAX, and let's just say there are lots of places where you can screw that up.

Phew, glad that's over.