Map reverse charge to the supplier regime's category

[pmenendz]

Problem

Converting a Stripe invoice from a Polish supplier billing an Australian customer failed with:

422: invalid-category: 'GST' not defined in regime

The invoice's regime comes from the Stripe account country (PL → Poland). Stripe reports the line tax from the customer's perspective — an Australian GST rate with taxability_reason: reverse_charge. The reverse-charge branch kept that GST category but stamped it onto the supplier's regime (PL), which only defines VAT, so GOBL rejected the combo.

Fix

A reverse charge is issued from the supplier's side: the supplier charges no tax and the customer self-accounts, so the combo must sit in the supplier's regime. Resolve the category accordingly:

• keep the category Stripe reports when the supplier's regime already defines it (EU → EU stays VAT);
• otherwise fall back to the regime's VAT category (EU → non-EU, e.g. PL → AU);
• if the regime defines neither, record the Stripe tax as-is rather than emit an invalid combo.

The reverse-charge key and the regime country are unchanged. Applied to both invoices and credit notes.

For invoice.json (PL → AU) the lines now convert to VAT / PL / reverse-charge and the invoice validates (total/payable 87.50). An actual customer-country percentage (not a reverse charge) is unaffected and still maps to that country's tax via the existing foreign-tax path.

Tests

Added cases to TestFromInvoiceTaxAmountsReverseCharge and TestFromCreditNoteTaxAmountsReverseCharge covering a foreign category (AU GST) not defined in the supplier regime (PL) → VAT / PL / reverse-charge. Existing EU → EU reverse-charge behaviour (VAT / DE / reverse-charge) is unchanged. Full suite passes; go vet clean.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR fixes reverse-charge tax combo generation when Stripe reports a customer-side tax category (e.g. AU GST) that is not defined in the supplier’s tax regime (e.g. PL), by selecting a regime-valid category (prefer reported category when defined, otherwise fall back to VAT) for reverse-charge combos on both invoices and credit notes.

Changes:

• Adjust reverse-charge handling to map the category into the supplier regime when possible (or fall back to VAT if available).
• Add reverseChargeCategory helper to centralize the category-selection logic.
• Extend reverse-charge unit tests for both invoices and credit notes to cover foreign categories not defined in the supplier regime.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
lines.go	Updates reverse-charge tax combo generation and introduces a helper for selecting a regime-valid reverse-charge category.
lines_test.go	Adds reverse-charge test cases for supplier-regime category fallback (e.g. PL supplier with AU GST reported by Stripe).

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