The sidewalk sign said:
Rapid antigen & Fast PCR Covid-19 tests
If you go to their website, they have sites in 14 states + Washington DC. If you click on the shop that I passed (Silver Spring), they talk about antigen tests for $50 (30-minute turnaround), $195 for PCR Test Priority (less than 36 hours -- vs "same day"), and an unspecified price for 3-day results. It's the standard nasal septum test that I had taken the last two times (they make it clear they're not the pull-your-brains-out-your-nose test).