To expect an understanding of basic maths in shops?

Visited out local M&S food today. Special offers £10 for three instead of £4 each. Checking my bill on way out of shop noticed I'd been charged full price. After a lot of fuss managed to get someone to, reluctantly, agree to a refund.

That'll be £3.25 we owe you he said fiddling with till
No, you owe me £2.....more fiddling with till
That'll be £2.25 we owe you then still fiddling with till
No, £4x3=£12. So if it's 3 for £10 that's £12-£10=£2

So we owe you £3.25 then?

I give up and take the £3.25 because factoring in my time yes, they do owe me!!