### What
Creating a swap on an order with a discount leads to an incorrect difference due.
**Scenario**
- Create a store with minimum 2 products (Prod A, Prod B)
- Create a discount that only works for Prod A
- Create an order for Prod A with the discount applied
- Create a swap between Prod A and Prod B
**Expected outcome**
We would expect the difference_due amount to come out to the sum of:
- -1 * (price of prod a - discount applied to prod a)
- price of prod b
**Actual outcome**
Instead the discount is applied across both products when calculating difference due. This results in a total that is instead the sum of:
- -1 * (price of prod a - discount applied to prod a)
- price of prod b - discount on prod b ignoring the condition
### How
Adds `line_item.adjustments` to relations in cart retrieval prior to setting the difference_due to car total
Fixes CORE-361
**What**
Since the release of the Tax API the line item totals calculations on orders with gift cards have been wrong. To understand the bug consider the below order:
Region:
- tax_rate: 25%
- gift_cards_taxable: true
Order:
- applied gift card: 1000
- items:
- A: unit_price: 1000
- B: unit_price: 500
- Subtotal: 1500
**Previous calculation method**
1. Determine how much of the gift card is used for each item using `item_total / subtotal * gift_card_amount`:
- Item A: 1000/1500 * 1000 = 666.67
- Item B: 500/1500 * 1000 = 333.33
2. Calculate line item totals including taxes using `(unit_price - gift_card) * (1 + tax_rate)`
- Item A: 1000 - 666.67 = 333.33; vat amount -> 83.33
- Item B: 500 - 333.33 = 166.67; vat amount -> 41.67
3. Add up the line item totals: order subtotal = 500; vat amount = 125; total = 625
This is all correct at the totals level; but at the line item level we should still use the "original prices" i.e. the line item total for item a should be (1000 * 1.25) = 1250 with a tax amount of 250.
**New calculation method**
1. Use default totals calculations
- Item A: subtotal: 1000, tax_total: 250, total: 1250
- Item B: subtotal: 500, tax_total: 125, total: 625
2. Add up the line item totals: subtotal: 1500, tax_total: 375, total: 1875
3. Reduce total with gift card: subtotal: 1500 - 1000 = 500, tax_total: 375 - 250 = 125, total = 625
Totals can now be forwarded correctly to accounting plugins.
Fixes CORE-310.
