Swombat

It's worth pointing out that I'm in the process of throwing together a simple plugin, loosely based on ActiveMerchant (meaning, I stole a fair bit of code from there), to process Protx VSP Server payments.
 
I've called it Peeves (Protx Vsp Server => PVS => Peeves), and it's available on Github at http://github.com/swombat/peeves/tree/master . Feel free to make use of it for your own ends (and/or copy, adapt, upgrade the code, whatever).
 
It's not functional yet, but it will be within the next week.
 
It doesn't have any tests... I think it's too simple in scope to be worth the overhead of testing.
 
One last note - why not use ActiveMerchant? Well, because ActiveMerchant relies on the assumption that we'll be capturing credit card details ourselves and sending them over to the payment gateway for processing. Within the Protx world, this is known as "VSP Direct". This is great for many websites, and certainly provides a good user experience (well, so long as they trust you), but it does submit you to the PCI DSS compliance standards, which are a requirement for any website that handles credit card details (even if you don't store them in your own database). On top of that, it also means you need to implement 3D Secure yourself - so you can't even guarantee that you'll be able to keep the user on your site and avoid messy redirects.
 
I don't want to have to deal with PCI DSS at this point in time, nor do I want to write any 3D Secure code - I have enough other things to worry about. Therefore, VSP Server makes more sense. Since ActiveMerchant doesn't directly support this (and it doesn't really fit within the API of ActiveMerchant), it seemed easiest to just extract a simple plugin to deal with this.

Update: I couldn't do it, I just couldn't. Writing code without specs just feels wrong. Adding in RSpec now.