So, is there no job called "transponster"?
Native speaker’s answer
Rebecca
No, "transponster" is not a real job and it is not a word. The only reason Rachel guessed this here is because Monica thought that Chandler's job was called something like "transponding", which is also not a word. The suffix "-ster" can be put at the end of a noun to indicate that someone is associated with doing something, so "-ster" is often used to refer to job titles which is why Rachel guessed "transponster" in this case.