I think the weight of the bag is pretty negligible, though you could always ask them to take out a veggie to weigh (they may have to anyway to get the code). I'd be curious to see the difference, but the flour sacks are really light so I'll bet it's not much of a difference (at least to the sensitivity of the scales).
Cool, thanks. I feel better about using these, as I have wanted to do something like this for a while. I am definitely giving it a try now! I haven't worked with the flour sack fabric, it looks more substantial in the pics than it sounds like it really is.