Worked on the medusajs/medusa repository to enhance tax and pricing accuracy within core cart workflows, focusing on backend business logic using TypeScript. Addressed a critical bug by ensuring gift card line items are excluded from tax calculations during cart tax recalculations, and preserved original pricing amounts when sales are stacked on overrides. The approach emphasized targeted integration testing and maintained performance by confining changes to calculation paths, avoiding schema or API modifications. Expanded test coverage validated both tax and pricing scenarios, resulting in more reliable checkout experiences for EU stores. Demonstrated skills in API development, backend development, and pricing strategy.