The shape rules
| Shape | Minimum | Maximum | Notes |
|---|---|---|---|
| Card (postcard price) | 3.5 x 5 x 0.007 in | 4.25 x 6 x 0.016 in | Unfolded paper or cardstock |
| Letter | 3.5 x 5 x 0.007 in | 6.125 x 11.5 x 0.25 in | 3.5 oz max; over 6 x 4.25 must be at least 0.009 in thick |
| Flat | Bigger than a letter in at least one dimension | 12 x 15 x 0.75 in | Must be flexible, rectangular and uniformly thick |
| Parcel | Larger, rigid or lumpy pieces | Priced as packages |
A letter also has to keep its length divided by height between 1.3 and 2.5, or it pays a nonmachinable surcharge. Square envelopes are the usual victims.
The formats
Postcard. One card, printed both sides. Cheapest to produce and read without opening. Standard size mails at postcard price; larger cards mail as letters or flats.
Letter in an envelope. The format for longer arguments, personal appeals and anything that should feel private. Costs more to produce: printing, folding, inserting, sealing.
Self-mailer. A sheet folded and sealed to mail without an envelope. More panels than a postcard, cheaper than a letter package. Mails as a letter or flat depending on its folded size.
Catalog or booklet. Bound pages. Usually a flat. Heavier pieces pay more, so weigh a sample early.
Large envelope. A flat-size envelope holding a brochure, samples or a packet. Feels substantial and costs more to mail.
Dimensional mail. Boxes, tubes and lumpy items. Hard to ignore and priced as parcels. Use it sparingly.
Questions readers ask
What's the most common direct mail format?
Postcards. They're cheap to print, need no envelope and are read without opening.
Why are square envelopes more expensive to mail?
A letter must have a length-to-height ratio between 1.3 and 2.5. A square one doesn't, so it pays a nonmachinable surcharge.
Is a folded mailer a letter or a flat?
It depends on its folded size. Within letter limits, it's a letter. Larger than that, it's a flat.