Lump-sum fallback when divided line price can't round-trip

[pmenendz]

Summary

• For each invoice/credit-note line, after computing price = line.Amount / quantity at currency-subunit precision, verify that quantity × price round-trips back to line.Amount. When it doesn't, collapse the line to (quantity=1, price=line.Amount) so the GOBL tax base reconciles with Stripe exactly. Tiered billing takes the lump-sum path unconditionally (no honest single per-unit price across tier breakpoints). Zero-quantity zero-amount lines keep the readable (qty=0, price=Price.UnitAmount) display.
• The trigger in production is Stripe's transform_quantity (e.g. divide_by=50, round=up): on a real PL invoice with Quantity=751, Amount=3520.00 PLN, the old 3520/751 = 4.69 rounded down, then 751 × 4.69 = 3522.19, inflating the line by +2.19 PLN and leaving a rounding: -2.69 PLN residue at the invoice level. The fix eliminates the residue.
• The Stripe-supplied item description (e.g. "751 users × Additional Active Users (at 220.00 zł per 50 users / month)") is preserved verbatim on lump-sum lines, so the per-unit narrative is never lost — only the synthetic per-unit price the data wouldn't support.

Test plan

• Added unit tests covering: clean per-unit, transform_quantity, proration mismatch, tiered billing, zero-quantity zero-amount, zero-quantity nil-price; plus credit-note clean / non-reconciling / zero-quantity variants.
• Regenerated examples/stripe.gobl/out/gobl_quantity_rounding.json golden — line 6 (metered 204 × 147 ≠ 30000) collapses to lump sum; the prior totals.rounding: 0.12 residue disappears.
• Verified end-to-end against test/invoice.json: totals.payable == "17724.30" matches Stripe's reported total exactly, with no rounding field.
• go test ./... green.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR updates Stripe→GOBL line conversion to ensure line tax bases reconcile exactly with Stripe totals by falling back to a lump-sum (qty=1, price=line.Amount) representation whenever a derived per-unit price cannot round-trip back to the original line amount (notably with transform_quantity and similar cases). It applies the same round-trip logic to credit note lines and refreshes tests and a golden example output accordingly.

Changes:

• Add round-trip verification for derived per-unit prices; fall back to lump-sum when qty × price != amount (invoice + credit note).
• Treat tiered invoice pricing as lump-sum unconditionally; preserve readable display for (qty=0, amount=0) invoice lines when a unit price exists.
• Add targeted unit tests and regenerate the gobl_quantity_rounding.json golden output.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 1 comment.

File	Description
lines.go	Introduces (quantity, price) resolution with round-trip validation and lump-sum fallback for invoices and credit notes.
lines_test.go	Adds unit tests covering clean per-unit, transform_quantity/proration mismatches, tiered billing, and zero-quantity edge cases.
examples/stripe.gobl/out/gobl_quantity_rounding.json	Updates golden output to reflect lump-sum fallback and removal of invoice-level rounding residue.

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