Over a two-month period, contributed to the invopop/gobl repository by delivering four features focused on regulatory compliance and invoice processing. Work included enhancing Polish Tax ID validation logic and adding support for Type T tourism invoices, ensuring robust data validation and adherence to local standards. Leveraging Go and JSON, implemented tourism-code rules and migrated tax mapping to improve maintainability. Expanded test coverage and collaborated on cross-repository integration, including ARCA integration and documentation updates in docs.invopop.com. Emphasized backend development, compliance, and clear documentation, resulting in reduced tax compliance risk and streamlined workflows for both Polish and non-resident tourist invoicing scenarios.