There is a bin for collecting used bags, including Post bags, at 7th and A St SE! Some people might think about putting up similar bins in the parks where dogs go. I think that bags shouldn't be put in other bags, but rather should be put in the bin individually. In any case, I'm giving my bags to my neighbor Dan. Other neighbors said that they would gladly take the bags too. Ask around and see if someone wants your bags.
One cold night, I went around to the various parks and found that kind souls provided new bags for dog owners.
What about creating a bin for used Post and other small bags? Of course, there is always another alternative, reported by Mr. Green: "If you are connected to a municipal sewage treatment plant, an alternative to sending dung to the dump is to buy a pooper scooper (or fashion your own by cutting out the bottom and part of the sides of a plastic jug to make it into a scooper). Scoop up the dog excrement, take it home, and flush it down the toilet." Well, that's all I'm going to say about bags and dogs! For now, maybe just more bins for used Post and other small bags?