So sorry you had this bad experience with them. And if the store people are bad about correcting their mistake, that's the only chance they should ever get for your business again.
When I'm having fabrics cut, I watch the cutting table like a hawk. I've had to gently correct the cutter at times when I see them about to make an incorrect cut. Usually it's because there is a lot of activity in the place, several people yakking with different conversations, etc. But I try not to get sidetracked from that and keep my eyes on the table. At the register, I do the same as they ring each item up and it appears on the thingy on top of the register. If the number doesn't seem reasonably correct, I stop them right there and ask them to check it. Like you, I've been charged full price for stuff that should have been on clearance (40% off).
I hope things work out well with this shop, but if not, don't darken their doorstep again and tell others why.