afaik...
Pratinhas bus goes to Presidente Vargas.
Marex bus goes to Praca de Republica.

The airport is only about 12 km from the city, so even a taxi isn't likely to be more than maybe U$20 or so.

If you have time in your schedule, I'd suggest spending a few days in charming Sao Luis while you're up there in the general area. Lots to do and see.