Surprise, surprise, this one's set in a filling (read: gas) station. And no, the setting does not shift at all during the course of the poem. In fact, this poem is entirely structured around setting. It's pretty much the most important thing we can pay attention to.
Where this filling station is—a city, the country, a foreign country—is impossible to say; when this poem takes place might be easier to estimate. We're betting it's probably before huge chains of car service stations started popping up. This is not a truck stop on the Interstate, complete with fourteen bathroom stalls—one for each kind of soda they have at the fountain. But we don't really know that either.
In fact, all that seems to matter to Bishop for this particular poem, is that we're transported to this particularly greasy, family-run filling station, and that we don't move a muscle until the poem is finished painting its endearing, albeit grimy, portrait of the place. We get every detail, and no generalizations.