fix: fixes #307

[Naoray]

Problem

When applying a percentage coupon (e.g., 50% off), the discount was calculated from the total price including tax (e.g., 50% of €181.50 = -€90.75) instead of the subtotal excluding tax (50% of €150.00 = -€75.00). This led to:

• Incorrect subtotal calculations
• Incorrect VAT/tax calculations
• Incorrect invoice totals (or coincidentally correct totals with wrong breakdowns)

s. #307 for detailed description of the issue

Solution

Calculate unit price of the coupon using $item->getSubtotal() instead of $item->getTotal()