Looking at the sample that you provided, everything looks in
order. The Unit Price is the price for one unit. The Extended Price
is the quantity times the unit price. I can't see on your example
where this is not the case.
If you mean something else, would you mind providing more
You see that water CRISTALINE is 1 euros so if you check
quantity 6 as in the INVOICE you will have 6 euros.
In the invoice (at the page 2) you have 36 euros ! The invoice make
6 by the quantity of 6 so 36 that's not correct !