Both. The verse says “good works” and doesn’t rank them, so the cashier interaction and the gospel conversation are the same light at different wattages.
Matthew 5:16 is Jesus saying your good works should be visible enough that people trace them back to God and give Him the credit. Patience in traffic earns you credibility for bigger conversations later. The key phrase is “that they may glorify your Father.” Treat the cashier well and be ready to explain why.