This commit fixes the inconsistent calculation logic between order preview
and actual order creation, which caused balance payment failures when using
coupons.
Changes:
- Standardized fee calculation order in both preCreateOrderLogic and purchaseLogic
- Moved gift amount deduction after fee calculation to ensure correct total
- Removed premature gift amount deduction before transaction in purchaseLogic
- Gift amount is now only deducted within the database transaction
The calculation order is now unified:
1. Apply coupon discount
2. Calculate handling fee based on post-coupon amount
3. Deduct gift amount from total (including fee)
This ensures the preview amount matches the actual payment amount.
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