BMS changed their charging system a few months ago. When I got the first bill from their new system, it was immediately obvious to me, by comparing it with my previous bill, that it was very wrong.
For example, in the past, I've been paying them about £50 per month in VAT charges, and now, suddenly, it was £5.
So I phoned them, and told them about that, and the other two obvious errors.
It took them about three months, and then they agreed with me about the VAT, and sent me a humungous refund on that (going back for years), which (of course) went straght to HMRC. But also a "sorry about that" compensation payment, plus "lost interest", which was nice to have.
But they didn't do anything about the other two issues.
Today, I got two letters from them.
One of them was to tell me that "due to a technical issue" they forgot to biill me £20 management fee per month. They aren't going to charge me for the back payments.
The other letter was to tell me that they've been charging me 1% for transactions, plus £0.10 per transaction, and it should have only been the 1%. So they are going to refund me a four figure sum for this cockup.
Win! Win!! Win!!!
I'm not going to complain about this, or make any suggestions to them about it, but I will tell you, confidentially, and hoping that you dno't mention it to them, make a modest proposal.
Test your software before you inflict it on your customers. Do a trial billing run, compare the amounts with what you charged last month, and when you notice that an item is twice as big as it was last month, try to work out why.
And some advice for their customers. I feel sure that I'm not the only one who has had the VAT cockup, but if you're also affected, then you should complain, and ask for compensation and lost interest. And as a general rule - compare your bill for the month with last month. Because they obviously don't.