As the climax to Avisa's reply to her suitors she breaks into a song that the author tells us has been set to the tune of Fortune.
Fortune or otherwise known as Fortune My Foe was a popular Ballad but the author of Willobie His Avisa had hundreds maybe thousands of other tunes to chose from so why has he picked this specific tune???
Following the death of Penelope Rich's father Walter Devereux a ballad was written for him called Essex's last goodnight. This was specifically set to the tune of Fortune.
Penelope Rich would definitely have had an emotional connection with this tune the author knows this and this is why he has selected it. We also notice that an alternative word for fortune is Riches.