I would like to have the following scenario in Point of Sale:

1. One item should have the price of let say $0.5

2. 10 items should cost $4

3. 11 items should cost $4.5 until you buy 20, then it should cost $8.

I've tried to achieve that with formula, but as soon as I set the minimum amount it always set amount to 10 in POS. It seems to be a very basic feature, but I fail to find a solution for it.

Any help will be highly appreciated.