I think they may crinkle up your clothes, so when the customer takes them out of the packet, their appearance won't be as nice.

I don't think they always cryovac dead flat, so there is no guarantee that they will always be under the 20mm - i.e they may be kind of wavy iykwim. So whilst each part of the package may be under 20mm, when it is laid out on a table or in the envelope, some bits will be higher with space underneath and other bits will be lower with the space below. And once cryovaced, they don't tend to be very flexible or pliable, so I doubt you could squish them flat iykwim. They're pretty rigid.

I don't know cost on domestic bags, I only use food grade bags on a commercial machine for food. (because I cryovac food) I pay about 4c each for the small ones I buy - but have a look on eBay heaps of competitive listings - don't know the quality though.. Bag cost and availability may also depend on the machine you buy, I don't know if the domestic ones are a standard size.

Would a sheet of cardboard and gladwrap do the trick? Stiffer envelopes? Cardboard mailers (eBay search 16mm cardboard mailers - a huge selection of sizes)

How many items would you need to post to recoup the cost and maintenance of a cryovac machine? Will the machine last long enough to produce those quantities? (rhetorical questions - even if you give an answer, I won't be able to advise you LOL)

May I suggest that you take a few items down to your friendly butcher and ask him if you can have a go on his machine to see how the products turn out, so that you can judge for yourself?