Those three questions place any stack of paper on the right service. Page count alone decides almost nothing.
The four-rung ladder
| Tier | Carries | Published speed | Fits when |
|---|---|---|---|
| First-Class letter | Folded pages in a standard envelope, letters capped at 3.5 oz | 1–5 days | A contract copy, a signed form, anything that tolerates folding |
| First-Class flat | A large envelope up to 13 oz | 1–5 days | Certificates, transcripts and photos that must stay unfolded |
| Priority flat rate envelope | Whatever the envelope closes over, one price to any state | 2–3 days | Heavy stacks, long distances, or a tracking number wanted |
| Priority Mail Express | Its own flat rate envelope, or by weight | 1–3 days by 6 PM | Filings and closings with a calendar date attached |
The jump that surprises people is the second one. Postage on letters and flats is bought by the ounce, so a thick unfolded packet can wander past what the Priority envelope charges regardless of weight — USPS words the flat rate pitch as paying the same for mailpieces “up to 70 lbs going anywhere in the U.S.” Once a stack outgrows 13 oz, the flat rate envelope usually stops being the premium option and becomes the cheap one.
Visibility and proof are separate purchases
A number to watch
The Priority Mail page lists USPS Tracking among what the price includes, and the Express page does the same. The letter tiers sit outside that system — a stamp buys carriage, and the envelope travels unwatched.
A record to cite
Legal and tax paper usually needs evidence more than speed. Certified Mail bolts a mailing receipt and delivery record onto an ordinary letter; the fees and the electronic return receipt are priced out on the Certified Mail guide.
The tax-season special case
Returns sent near a deadline lean on the postmark, and the way filers use certified for that is its own subject — covered in certified mail for tax returns, including what the receipt does and does not prove.
Original documents that cannot be replaced — passports, deeds, stock certificates — are the one case where the calculus changes entirely, and the postal answer is Registered Mail; the certified against registered comparison explains why they are different products.
When documents become a parcel
Case files, archive boxes and print runs stop being mail and start being freight in miniature. A banker box of paper weighs in the neighborhood of 30 lb, which prices like any other 30 lb parcel — by weight, box size and distance, across every carrier at once. That comparison is what the calculator is for, and it takes a description and two ZIP codes rather than a service name.
Sources
- USPS, First-Class Mail — the 3.5 oz letter ceiling, the 13 oz flat ceiling, and 1–5 day delivery. Read 2026-08-09.
- USPS, Priority Mail — 2–3 day delivery, the flat rate any-state pricing, and included tracking. Read 2026-08-09.
- USPS, Priority Mail Express — 1–3 day delivery by 6 PM, every-day operation with limited exceptions, and the Express flat rate envelope. Read 2026-08-09.
Common questions
What is the cheapest way to ship documents?
A stamped First-Class letter, for anything that folds into a standard envelope and stays under the 3.5 oz letter ceiling. USPS publishes 1 to 5 day delivery for the class. Current postage figures live on our First-Class Mail guide.
How do I mail documents that must not be folded?
Use a large envelope, called a flat, which First-Class Mail carries up to 13 oz per the USPS page read 2026-08-09. Add a rigid backer inside, since flats travel with machine-sorted mail and nothing about the class promises gentle handling.
When is the Priority Mail flat rate envelope worth it?
When the stack is heavy, the distance is long, or you want a tracking number without arithmetic. One published price covers the envelope to any state, USPS lists the service at 2-3 days, and USPS Tracking is included.
What does overnighting documents involve?
Priority Mail Express, which USPS describes as 1-3 day delivery by 6 PM, running every day all year with limited exceptions. It sells a flat rate envelope of its own, so a fat filing costs the same as a thin one.
How do I prove the documents were sent and received?
Certified Mail puts a mailing record and a delivery record on an ordinary letter, which is why attorneys and tax filers default to it. The current fees and the return receipt options are broken out on our Certified Mail cost page.