How does return to sender work?

[One]

I've now got my 3rd letter from the student loan company asking for proof of work. I've filled them in each time before. They require a stamp to send which I think is annoying. THey want info from me but I have to pay to give it to them?

I don't really want to waste another stamp only to recieve the same request 3 months later. How can I address the envelope so it gets sent to them and they have to pay for the postage?

 

[One]

eurgh student loan company sucks. Might request they take the cost of 3 stamps off my remaining balance. Out of principle obviously and to cause agro to some poor sod working for them.