Wrong calculation of fix amount discount

Problem/motivation

When generation promotion coupons with FIX price amount discount, module calculate discount % from unit price instead of total item price (for all quantities).

Steps to reproduce

• create discount with fix amount per item
• add to order item with multiple quantities
• add discount coupon
• Submit order
• check superfaktura administration and see wrong discount in that item

Proposed solution

for fixed amount discount calculate % from total item price