On the Over-Complication of Life

Why are cash tills in charity shops so complicated?
I went into the Red Cross shop the other day, and saw a lovely summer dress (yes, I'm being optimistic that we're actually going to have a summer!). It didn't have a price tag on it, so while the manager went to check how much it would be, I found a jumper I liked as well.
And then they tried to ring the two purchases into their whizzy, modern, over-complicated till. And got it wrong. Twice. So then they had to fill in more paperwork to show that they'd got it wrong (because it all gets checked). And then they tried again. And finally, I had my purchases, and the correct change - and there were three people clustered round the till, trying to make sense of the thing.
I understand that the charities have to guard against theft (hence the auditing of mistakes) but why make it so complicated in the first place? They must know that they are depending on volunteers, who often don't have a background in retail, and are doing it only a few hours a week - so if they are trained to use the till once, it's easy to forget it by the next time they have to use it in that way. And the tills must cost quite a lot to start with.
The manager said that they were waiting for a simpler till to come (it's far more complicated than the one I use at work). It's a pity they can't go back to a hand written book and a tin box under the counter!