But the van at Sweetbay has limited services.
For Happinow, who I gather will be a resident of the Village of Sanibel, your closest bet is, indeed, going to be the USPS facility in Fruitland Park, just a couple/three miles east on CR-466-A.
There is a "UPS Store"-style facility in Colony Plaza, but it's not a Post Office in the sense that I think you meant!
SWR