Hey Pete - nice to see you back again. Isn't it a pain when the day job gets in the way Roll on Organized manic chaos ...
Testing is a brave new world and I tend to take my lead from the clients company testing policy.
Certainly the one key reg to bear in mind is that you can't mix live and test subject data. Keep one set-up for all financial tests. The moment you get a real client with data stored in the db the implications of testing change.
Check with the processor how they expect test purchases to be performed. Given the security implications of getting it wrong, these guys are very happy to help (in most cases).
You need to know does the client require every item to be test purchased or are they happy to see that ZC takes money. If it's the later get a signed release
I try to ensure that test purchases are made on any items that have price variants and / or delivery options. It's good to prove that things are working as expected. This is essential for international transactions.
In general as long as you can show that you have tested the system well and there are no obvious gaffs you can start a few refund requests.
If that goes well go live and out of habit I make the first real transaction - again just to be sure. (you'll note I do this after having previously ensured the refund system does work )
Kev