HMPC: SQUARED

i'm not positive, but i think that in the US square postcards are considered non-machineable and require a surcharge over the 1st class (forever) stamp.

Yes, square envelopes need the extra non-machineable surcharge and the minimum size is 5" x 5". I assume this applies to postcards too, but couldn't find that specifically.

I read on the USPS website and confirmed at my local post office that the 6x6" square postcard is .49 cents to mail in the us. If you go over 6" it is considerd a "large envelope" rate piece by virtue of size. SO, don't use a postcard stam, use a full rate, first class for 1 ounce stamp (or first class Forever stamp).

PS: If it is a 6x6" envelope, then it needs the first class stamp PLUS a surcharge stamp (also forever stamps but I think they are .21 cents)

P.s. - postal rates decreased last month to $0.47 for first class, $0.34 for postcards, extra ounce = $0.21, and international down to $1.15. All forever stamps bought in each category will suffice. See a great visual drawing at LWA website for a reminder: http://16sparrows.typepad.com/letterwritersalliance/page/2/