Just call and say, "Hey, can you find those reusable bags and set them aside, so I can pick them up the next time we come? I don't have a big supply, so I like to hang on to them."
Some people might think that the actual bags aren't such a big deal, so just ask.
I had this once; I would put a bill to pay into our co-op treasurer's box, using a clean envelope w/ a post-it on top; he'd return the bill and check in one of his "salvaged" envelopes (window envelopes from other mailings).
After the 2nd time, i just wrote him a note and said, "Please return these in the envelope I give them to you in; I use those clean, new envelopes for mailing the check out to the vendor, and I don't want to have to get yet another new one out."
I'll admit to being a little peeved, because since my *new* envelope was rightthere with the bill, it did seem as though he was cadging it for his own use and substituting his "salvaged" envelope. All the time I've known him, he has been saving the window envelopes, etc., from his bills to use for this sort of purpose--which is totally fine. But then when my clean, new ones went out but DIDN'T come back, it felt like he was taking advantage of me.
So I just simply asked, straightforwardly, and bingo! The stuff I give to him comes back in the same envelope.