I think it would be vacationing, because it is the subject of the sentence. The only other option would be sewing but since that looks more like an adjective I think vacationing would be the right answer.
Answer: Nearly 90% of mail is junk mail, throwing it away is bad for the enviroment.
Explanation:
Because, it has nothing to do with the claim, the claim was about needing post offices, not what people do with their junk mail, and why it's bad for the enviroment