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Step 1: Choose Envelope or Postcard When mailing a letter or postcard, postage cost depends on the size and shape of the mailpiece. You can save money mailing standard sizes that fit through automated mail-processing equipment. Size & Weight Determine Price All envelopes must be flat. If your envelope cant fit through automated processing equipment it is considered non-machineable and costs more to send. Non-machineable items include lumpy or rigid envelopes, and those that have clasps, string, or buttons. Unusually shaped square or vertical envelopes will also cost more to send. Envelopes Envelopes must be made of paper. Envelopes must be rectangular to qualify for letter prices. Envelopes larger than letter-size will be charged package prices. Large envelopes must be rectangular to qualify for Flat Rate prices. Postcards Postcards must be rectangular to qualify for First-Class Mail prices. Oversized postcards will be charged as a letter or large envelope.