USPS Priority Mail Flat Rate is one of the most straightforward deals in domestic shipping: one fixed price, regardless of weight, up to 70 lbs. Ship a camera body or a pound of circuit boards for the same envelope price. The catch is the item has to fit inside the envelope — and the Large Flat Rate Envelope, at 12.5” × 9.5”, doesn’t leave much room for the box you were planning to use.

That gap between “almost fits” and “actually fits” is where this tip lives. The video is 67 seconds, but the setup is worth unpacking.

The shipping math

As of mid-2024, USPS Priority Mail Large Flat Rate Envelope runs about $10.40 at commercial rates, or $10.65 at the counter. A Medium Flat Rate Box — the next tier up when something doesn’t fit in the envelope — runs around $17. For items that weigh more than a pound and fit within a certain footprint, that’s a real gap. On the kind of regular-volume shipping you’d do selling electronics or camera gear, it adds up quickly.

The Large Flat Rate Envelope dimensions are 12.5” × 9.5” and flat — meaning the envelope itself has no depth budget. USPS’s rules say the contents can add some thickness and the envelope can be folded closed or left open at the short end, but anything in there has to be slim. A standard shipping box that fits the footprint but stands 2–3 inches tall won’t close inside the envelope.

The problem with standard boxes

Boxes are manufactured in standard increments. The cheapest box that fits your item’s footprint is almost always deeper than you actually need. A camera, a set of boards, or a tool that’s maybe an inch thick often comes packed in a 2” or 3” deep box because that’s what the manufacturer had, or what you grabbed from the recycling pile.

One inch of extra depth is the difference between flat rate envelope and flat rate box pricing. A box resizer gets you that inch back in about a minute.

What a box resizer is

A box resizer is a hand tool with a scoring/cutting blade mounted in an adjustable guide. You set the guide to the height you want, press the blade against the box wall, and run it around the perimeter. The blade scores and cuts the four walls at a uniform height. You fold down, tape the new edges, and you have a custom-depth box without needing a table saw, a straight edge, or much precision at all.

They’re a standard warehouse tool — Uline, ShipStation, and similar suppliers sell them, and generic versions on Amazon run $15–25. If you ship things with any regularity, the resizer pays for itself quickly: one use where you drop from a $17 box price to a $10 envelope price and you’ve covered the cost of the tool.

The technique

The video demos this on a camera body — something fragile enough to need a box rather than going loose in a padded envelope.

1. Assess the box. Close the box as if you’re about to ship it. Feel how much excess depth you have — in the video, it’s about an inch too tall to seat flat inside the envelope.

2. Flip it upside down. Working from the bottom gives cleaner cuts. The top has pre-scored fold lines and sometimes a manufacturer’s glue seam; the bottom is a cleaner surface for the resizer blade to track against.

3. Set and cut. Set the resizer guide to your target height — roughly one inch less than the current box depth. Run it around all four sides. Cut the corners with scissors or a box knife if the resizer doesn’t handle them cleanly.

4. Fold and tape. Fold the walls down to the new height, pad the contents, and tape up. Use good packing tape — if you’re trimming a box this tight, the tape job needs to hold.

The resized box should now sit flat inside the Large Flat Rate Envelope with enough clearance to seal it. USPS requires the envelope to close; if it can’t close around the box, it doesn’t qualify for flat rate pricing.

The gotcha: padding still matters

Flat rate envelope doesn’t mean fragile-item pass. The envelope itself provides zero impact protection — what you’re doing is using a box inside the envelope, which gives you foam, bubble wrap, or crinkle paper against the item. The tighter fit actually works in your favor because the item moves less inside the box, which reduces impact risk during sorting. But the padding still needs to absorb corner drops and the general mechanical abuse of postal handling.

For a camera body, bubble wrap or cut foam is the right call. For PCBs or small electronics, anti-static bubble wrap. For anything that can rattle inside the box even with padding, add crinkle paper to fill the void.

The other practical note: weigh the final package before you commit to the flat rate envelope. Flat rate pricing is capped at 70 lbs, which you’re not going to hit with a camera, but the envelope rules also require it to physically close and seal. Stuff a box that’s still slightly too thick inside and you’ll be at the counter re-packing — not a great situation.

When flat rate envelope wins

The envelope wins when your item is over about a pound and fits the dimensions. Below a pound, USPS First Class or Ground Advantage will often beat it on price. Above about 5–6 lbs, Priority Mail Regional Rate or Ground Advantage by weight may catch up. The sweet spot is roughly 1–5 lbs where flat rate pricing holds a consistent advantage over calculated Priority Mail.

For eBay or Reverb sellers shipping cameras, lenses, or small tools in the 1–4 lb range, the flat rate envelope is one of the more reliable cost controls available. The box resizer is what makes it actually usable for items that would otherwise need protective packaging.

References and further reading

• USPS Priority Mail Flat Rate — current rates, envelope dimensions, and weight limits
• USPS packaging guidelines — the rules on envelope closure, reinforcement, and what qualifies as a flat rate shipment
• Uline Box Sizer — the standard warehouse version; generic equivalents on Amazon under “box resizer” work for occasional use