LetterAgent
Mail physical letters by just asking your AI assistant.
USPS offers five PO Box sizes, from extra-small (3 by 5.5 inches) up to extra-large (12 by 22.5 inches), with most boxes about 15 inches deep. The smallest holds roughly a dozen letters; the largest swallows packages. Not every post office stocks all five sizes, so availability varies by location.
| Size | Inside dimensions (W x H) | What fits |
|---|---|---|
| Extra-small (Size 1) | 3 in x 5.5 in | About 10 to 15 letters and a few rolled magazines. Letters only; no packages. |
| Small (Size 2) | 5 in x 5.5 in | 15 or more letters, five rolled magazines, or one small flat-rate box. |
| Medium (Size 3) | 5.5 in x 11 in | Large envelopes and magazines lay flat; holds two small flat-rate boxes. |
| Large (Size 4) | 11 in x 11 in | Small and medium flat-rate boxes with room left for a stack of letters. |
| Extra-large (Size 5) | 12 in x 22.5 in | Multiple packages, including large flat-rate boxes. The business choice. |
The depth on most boxes is about 14.75 inches, which matters more than people expect: a medium box that is only 5.5 inches tall can still hold a stack of large envelopes piled flat toward the back. Dimensions above are the inside opening measurements published by USPS.
Start from the biggest thing you receive regularly, not the average. If the largest item is a magazine that arrives weekly, a medium box keeps it flat and uncreased. If online orders arrive as packages, jump straight to large or extra-large, because anything that does not fit gets held at the counter or put in a parcel locker instead of your box, and counter pickup means standing in line during business hours. Renters of small boxes learn this quickly: the extra-small is honest about being letters-only.
Also weigh how often you collect. A box you empty every day can be one size smaller than a box you empty weekly. Letting a box overflow is a real problem: if it fills up regularly, the postmaster can require you to move to a larger box. Small post offices sometimes carry only the three middle sizes, so if you want an extra-small or extra-large, search availability on the USPS PO Box page before you visit.
Yes, through a feature called PO Box street addressing. When you opt in, your box gets a street-style address (the post office's street address plus your box number), and private carriers like Amazon, UPS, and FedEx can deliver there. Subject to availability at your post office. Without it, private carriers cannot deliver to a PO Box at all, since a PO Box number is not a street address they can route to.
Yes. Keep your signature on file with the post office and signature items are placed in your box or a parcel locker, so you skip the trip to the counter. If your signature is not on file, you will get a pickup slip instead. See our guide on certified mail to a PO Box.
Ready to mail it? Send your letter with LetterAgent: describe it to your AI assistant, approve the exact quoted price, and it is printed and mailed.
LetterAgent is a working name. Prices include print, postage, and our service fee. US prices in USD, Canadian prices in CAD (Canadian prices exclude applicable sales tax, added at checkout). Currently in private testing.
Need help? Ask your assistant: it can check a job's status any time. If it ever can't handle something: hello@getletteragent.com