Nothing's up with the title, really. Sorry to rain on your parade. But the fact of the matter is, Bishop often titles her poems after their main subject. This poem is about a filling station, so she titles it, "Filling Station."
A filling station is a place where people when to get gas or their oil changed in their car. Although this title is pretty straightforward, it's interesting that Bishop chose to call it "Filling Station" rather that "The Filling Station," because she goes to such great lengths to describe the individuality of the place, and each minute detail. It seems to us by the end of the poem that this isn't just any old filling station, but a particular one that has caught the poet's eye. But hey, it just might have more universal applications, so maybe that's why she dropped the article.
Or maybe she just forgot.