As long as the envelope follows the rules (readable address, postage), it's "acceptable"... you would not believe the kind of stuff that goes through the postal service. There are helpful things you can do to improve the chances of the envelope going through without a hitch. For clear envelopes, that would mean putting the address on a sticker, or somehow making sure that a barcode reader can read it (high contrast, like black lettering against white, neatly handwritten instead of messily scrawled).
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Clear envelopes
