Thanks for the great feedback, everyone! I'm still on the fence, so if you have an opinion, please share.
I drive a Prius and I'm going to be smug at you now. See: *SMUG*
Gosh cello, you just don't go into things half-assed do you?
Nope. I tend to do things full-assed. You can get into lots more trouble that way.

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I would think that the best method would be the most bio-friendly, even if it adds something to the cost, so be it. Reasonable efforts need to start somewhere.
That's kind of the way I was leaning...
Hmm...I wonder how many people would actually recycle these "tin tie" bags?
Not all tin tie bags are recyclable---sorry if I was confusing on that. "Tin tie" just refers to the style of bag---tall, square-bottom, with an attached metal tie you can fold over to hold it closed after you roll down the top. They're made out of all kinds of materials, and many of them are NOT recyclable.
The specific ones that I'm looking at are regular paper with glassine lining, so I'm pretty sure they would just go with "mixed paper" or whatever your local equivalent is, if you have one. (I live in NH where recycling is widespread, and I forget that it isn't like that everywhere... yet!)
Even if they got thrown away, the paper/glassine bags would biodegrade much MUCH faster than poly bags, so it would still be better for the environment.
Just a thought. If you have to special order them, isn't that more fuel/labor that you are using to get the product to you, instead of buying zip-close bags picked up from the store where you were going to go anyway?
Zip-close bags from the store are WAY more expensive than ordering by the case, so I would be ordering either way.
Another question: how fresh do these tin tie bags keep the product? Less fresh, more fresh, or just as fresh as zip-close bags?
Freshness is my biggest practical concern with the tin tie bags. If I had to guess, I would say they keep the contents either just as fresh or slightly less fresh than the poly bags. The tin ties wouldn't be completely air tight, but I think they would be pretty close to it as long as you roll the top and close them properly. So if we're talking relatively short-term storage, I don't think there would be much difference. If we're talking many months or years, the poly bags would probably keep it fresher. Though if the air doesn't get squeezed out of the poly bags every time they're opened and re-closed, I think their advantage is lost.
Too late to do that discreetly, since you've started this post
Nope, I've tried to be up front about everything. These days, any effort to conceal information is doomed to fail eventually, and I feel like most people prefer honestly and transparency. Don't get me wrong---I'm a designer and I work in marketing, so I understand the value of a well-crafted message, but it depends on the audience. This is a relatively close-knit community for an online forum, and it just wouldn't work to try to make some slick sales pitch. Besides, it makes my life easier to be able to bounce ideas around with my fellow ACers.
If your buyers are as eco-conscious as you are, they will reuse the Ziploc bag and recycle the envelope it came in...
As I sheepishly admitted, I don't reuse zip-close bags. I try to limit my use of them, but they are just such a pain to wash and dry... That's why I'm looking into other options.