Billing bug squash: five failing tests, four causes

Context

Welcome to payments. billing/ builds the invoices the dashboard shows; the rules are in README.md. Since last week's refactor five tests fail, from four causes. The code is new to you: run the tests, read each failure, follow it into the code and fix the cause. Every fix is small. Change the code, not the tests.

You have 40 minutes. The checks are graded one by one, so each failure you fix counts even if the rest is not done. It is open book: read the code and the Python docs freely. The round this is modelled on is usually done without an AI assistant, so try it that way.

The failing tests

• test_a_payment
• test_a_refund_reopens_the_amount_due
• test_invoices_do_not_share_lines
• test_overdue_invoices_in_order
• test_not_overdue_on_the_due_day

Acceptance criteria

• pytest tests/visible passes, with the tests unchanged.
• The README's rules hold beyond what the tests check: the hidden checks cover each failure more widely.

Notes

• Five tests fail on the current code.
• The hidden checks are grouped BS-1 to BS-4, one group per cause.
• Practice task in the format reported for Stripe's Bug Squash round: an unfamiliar repo, a red test suite, and small fixes found by debugging. Original StackYard code, not the actual question; not affiliated with Stripe.