Sanibel Island

Sanibel Island: United States Post Office (Zip Code 33957)

The post office is centrally located and easily reached from the bike paths.
650 Tarpon Bay Road
Getting there:
Follow Periwinkle until it ends at Tarpon Bay Road. Right on Tarpon Bay Road, and the Post Office is on the left.
Or, from the west, follow West Gulf or San-Cap to Tarpon Bay Road, and look for the Post Office just north of Periwinkle.

Captiva Island: United States Post Office (Zip Code 33924)

No, Captiva captives don't have to go all the way to Sanibel to buy stamps.
14812 Captiva Drive
Getting there:
Follow Captiva Drive, the main road through the main section of Captiva.