LetterAgent
Mail physical letters by just asking your AI assistant.
Home > Guides > Seal a package
Use wide packing tape, seal the top and bottom center seams fully, and run tape across the side edges in an H pattern so no flap can lift in transit. One strip of narrow tape down the middle is the most common failure: boxes burst along the seams when tape peels or flaps catch on sorting belts.
| Method | When it holds, and when it fails |
|---|---|
| H-taping with wide packing tape | Strongest common method; seals the center seam and both side edges |
| One strip down the center seam only | Fails most often; side flaps lift and catch on belts |
| Masking tape or cellophane tape | Too weak for parcels; peels in heat, cold, and humidity |
| Strapping or filament tape | Good extra strength for heavy boxes, on top of packing tape |
Use pressure-sensitive packing tape, at least two inches wide, made for sealing boxes. It sticks fast, stays stuck across temperature changes, and resists peeling when a box brushes against other parcels. Duct tape is a poor substitute: it looks strong, but its adhesive softens in heat and lets go over time, and some carriers will not accept it. Masking tape and ordinary cellophane tape are worse: both are made for paper and light jobs, not for holding a box shut through a sorting plant. For a heavy box, add filament tape, the kind with visible fibers running through it, over the packing tape for extra strength.
Start with the bottom of the box, since a weak bottom seal is what spills contents. Close the flaps, run one strip of tape down the center seam, and extend it a couple of inches onto each side of the box so the tape wraps around the edges. Then run a strip across each end of the seam, where the center seam meets the box edges, forming the crossbars of the H. Repeat the same pattern on the top. Press the tape down firmly along its full length, and run your hand over the edges to make sure no corner is lifting. A tape gun helps here: it keeps tension even and lays the tape straight, which matters more than it sounds.
Most seal failures come from the box, not the tape. An overfilled box bulges, and the pressure pushes flaps apart no matter how well you taped them. An underfilled box crushes when other parcels stack on top, and the crushed edges peel the tape away. Old boxes are a risk too: tape sticks poorly to dusty, damp, or previously taped cardboard, and a box that has already done one trip may have weakened seams. If you reuse a box, peel off old labels and old tape first, and check that the flaps still meet cleanly. Fill empty space with crumpled paper or bubble wrap so the box holds its shape, but do not pack it so full that the flaps will not close flat.
Do not run tape over the shipping label. A taped-over label can glare under scanners, and a barcode the scanner cannot read means a parcel that gets diverted for manual handling. Place the label on the largest flat side of the box, away from seams and edges, and keep it flat and fully visible. If you use a clear pouch for the label, make sure the pouch itself is sealed flat and does not wrinkle across the barcode. The same rule applies to any customs form or battery marking: keep markings flat, uncovered, and on a side that will not be taped over.
When you ship with a LetterAgent label, the same sealing rules apply. You ask your AI assistant, get a per-label quote, approve the exact price, pay once through Stripe, and receive a Canada Post Xpresspost label PDF for a parcel within Canada, up to 5 kg. Print the label, place it on a flat side away from the taped seams, and seal the box with the H pattern before drop-off.
Ready to ship it? Get a shipping label with LetterAgent: describe it to your AI assistant, approve the exact quoted price, and get your label.
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