USPS letter size gauge

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A handy device for checking letters before you send them. Anything that doesn't fit through the slot doesn't qualify as a letter and will need extra postage, going either as a large envelope ("flat") or as a parcel. See the [Domestic Mail Manual (DMM) section 101](https://pe.usps.com/text/dmm300/101.htm) for more information on all this. Minimum and maximum width and minimum length are marked on the slot, while the maximum length is twice the distance between the left edge and the mark on the bottom. This was done so the gauge will fit on most print beds. Note that **the STL is in inches, not millimeters, so scale up by a factor of 25.4** if it's really small.