Legacy debugging set: seven checkout helpers, one bug each

Context

This set follows the debugging section candidates reported in Amazon's online assessment from about 2019 to 2023: seven short functions, each meant to do what its docstring says, each with exactly one bug. That section used C, C++ or Java; this set is in Python. Recent Amazon assessments use a different format, so treat this as practice for the skill: finding a fault fast against a written spec.

Each item is graded on its own, so every one you fix counts. The bug is in the function's code, never in its docstring; the fix is usually a line or two.

Items

Item	File	Function
Q1	checkout/cart.py	cart_total
Q2	checkout/discounts.py	apply_discount
Q3	checkout/shipping.py	free_shipping
Q4	checkout/stock.py	count_in_stock
Q5	checkout/pricing.py	cheapest_item
Q6	checkout/status.py	order_status
Q7	checkout/payments.py	split_payment

Notes

• Run pytest tests/visible. Seven tests fail on the current code, one per item.
• Practice set modelled on candidate reports of the debugging section in Amazon's earlier online assessment. Original StackYard code, not the actual questions; not affiliated with Amazon.