ATTN: Card Makers - Question about Envelopes

[MadilynnsMomma]

I am planning on making my own Mickey Mouse head invitations for my daughter's upcoming 2nd birthday and I have already figured that they will fit in a homemade 6x6 envelope. Today I was told by the cashier at my local scrapbook store that the post office frowns upon square envelopes...

Any insight into this?

TIA!

 

[sumlthomp]

I'm pretty sure they charge additional postage to mail square cards.