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USPS security rules require stamped mailpieces weighing more than 10 ounces or thicker than half an inch to be presented in person at a post office counter. A piece that big cannot be dropped in a collection box: if you do, it is pulled from the box and returned to you with an explanation instead of being mailed.
The rule applies to mail bearing only postage stamps. Any stamped piece over 10 ounces, or thicker than half an inch at its thickest point, must be handed to a retail clerk or carrier at the point of mailing. The clerk's acceptance is the security checkpoint: a human sees the item, confirms the postage, and puts it into the mail stream. A collection box has no checkpoint, so oversized stamped pieces are intercepted downstream and sent back.
| Situation | What happens |
|---|---|
| Stamped, under 10 oz and under half an inch thick | Normal mail: drop it in any collection box |
| Stamped, over 10 oz or over half an inch thick | Must be presented at the counter; box-dropped pieces are returned to sender with an explanation |
| Metered or online postage, any size within service limits | Exempt from this rule: the postage was purchased through a traceable account |
| No return address and box-dropped | May be held or disposed of rather than returned, since there is nowhere to send it back to |
It is a security measure, introduced after the 2001 anthrax mailings. Anonymous stamped mail dropped in a box leaves no record of who sent it, and the weight and thickness thresholds were chosen to catch the pieces most likely to contain something other than paper. Metered and online postage are exempt because the postage account behind them identifies the sender: the audit trail already exists, so the counter presentation adds nothing.
Something else, and the distinction matters. This is not a postage-due problem: your envelope can have exactly the right postage and still come back. It is also not the nonmachinable surcharge, which is a fee for pieces the sorting machines cannot handle. The counter rule is about how the piece enters the system, not what it costs. A thick envelope with perfect postage, dropped in a box, is returned for the rule violation alone.
Take it to the counter. The clerk weighs it, checks the thickness, confirms the postage, and accepts it into the stream. If you mail thick pieces often, consider switching from stamps to online postage: metered and PC postage are exempt from the counter-presentation rule, so a heavy envelope with a printed label can go in the box (or be handed to your carrier) like normal mail. The exemption is one of the quiet advantages of printing postage at home.
In Canada, there is no identical weight-based counter rule, but Canada Post has its own acceptance practices for oversized or overweight lettermail: pieces beyond lettermail dimensions move to parcel services, and counter staff will route them accordingly. When in doubt on either side of the border, the counter is the answer.
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