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We have both mail and postage. The tag wiki for postage says that it's about post (a.k.a. mail) in general, not just about postage (stamps and pricing), which sounds sensible for me. I think that
I'd say mail should be the main tag but this may be the Rightpondian in me speaking.
Personally I agree with renaming postage to post and synonymising post into mail so that the latter becomes the main tag.