In The Merchant of Venice, what did Portia do after receiving Doctor Bellario’s reply to her letter?
The reader is not precisely told what Portia does after receiving Bellario's reply to her letter. This is because we are not with her when the reply comes. Shakespeare jumps forward in time to the courtroom in Venice so the reader is kept in suspense as to the affect of the letter.
In Act III, Scene 4 Portia dispatches Balthasar with a letter to Doctor Bellario, then she and Nerissa commence a twenty mile journey to Venice by coach and ferry. The next time we see Nerissa and Portia it is in court in Venice and the surprise is sprung on us as well as Antonio and Bassanio that Portia and Nerissa are disguised as Doctor Bellario and clerk (of course, Antonio and Bassanio only find out the surprise in Act 5, Scene 1).
The action of the play reveals that what Portia did after receiving Bellario's letter was to continue to Venice; put on the clothes of a learned Doctor of Law; attend to the legal counsel contained in the letter; attend court and...as we know...win the case in favor of the Christians, even though she made a strong appeal to include Shylock in a winning result.