“Back and forth “ is a phrase and it literally means “there and back” between 2 things/2 places. So in this article they are expressing their unhappiness of having to pay for a postage stamp every time they send a letter between them 2. Rather then corresponding via email, which is free ( and faster🤣)

In the context of the sentence, it is referring to the fact that emails have already been exchanged between Albert Murphy and his pen pal. "Forth" refers to the emails sent by Albert and "back" refers to the responses Albert received from his pen pal. Hope that helps.

back and forth - writing each other many many times over and over again.