Most tape does the job on a lightweight envelope. A flat rate box packed with thousands of bulk cards is real weight in transit, and the seam that isn't glued at the factory is the one most likely to come apart under handling (the exact packing method is covered in the cheapest way to ship bulk).
What to actually look for
- A minimum 2-inch width. Narrower tape needs more overlapping strips to seal the same seam, which wastes more tape than it saves
- Real adhesive strength rated for shipping, not office or craft-grade tape, which peels under sustained weight and vibration
- A dispenser with a built-in cutter if taping more than a handful of boxes a week. Hand-tearing tape gets slow and messy at any real volume
Where to actually put it
Three strips minimum on a heavy bulk box: one straight across the main seam, then a cross pattern reinforcing it, plus the seams folded over and taped down flat rather than left as loose flaps. The one seam that isn't glued by the box manufacturer gets extra reinforcement, two or three additional strips, since that's statistically where a heavy box is most likely to split open in transit.
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Minimum tape width
Below this, sealing a real seam takes too many strips.
in2
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Strips on a heavy bulk box
Main seam plus reinforcement on the weak corner.
min3+
Shop packing tape on Amazon → (paid link) See the full box-packing method →