Check with a counter service place where you are staying. I had heard about it but was unsure about how it worked too, so I asked the night before we checked out. You can trade one table service for 1 counter plus 2 snacks OR 1 counter plus 1 mesh bag. The mesh bag is not bad. It's a blue drawstring bag measuring about 18"x12" with Walt Disney World on it. I will try to post a pic for you later. At the Poly, the people at Capt Cooks were very helpful in explaining it all. This was the only place in our resort that you could use counter service, but snacks could either be used there or a snack store upstairs. This is what we had left on our last day......
1 counter
2 table - we got 2 counter plus two coupons for 2 snacks OR 1 mesh bag
6 snacks
We used our three counter service meals for breakfast, then took our coupons to the snack store and got 6 snacks for the ride home plus used the coupons for 2 mesh bags.
I tend to over-explain things....I hope I didn't confuse you! I would check at a counter service restaurant in the resort you're in. Checking with the front desk didn't help me any. She acted like she had never heard of trading and told me I would have to check specifically with the place I wanted to trade with. However, the front desk can print you out a list of credits you have left throughout your stay if lose track.
Also, I've heard that you can trade counter service meals for several snacks if you can use those better. We didn't do that since we already had so many left but that would be great for people who don't want a big meal before they leave, but would like extra drinks, chips, fruit, etc. for the trip home.