Effective adhesive material to splice cardboard shipping boxes?
The boxes from USPS are of consistently flat and relatively thin material, while the adhesive strip is also quite thin. As such, this material would not work well on coarser surfaces such as corrugated cardboard, especially as that material is not particularly flat.

Hot melt glue is thicker in general, certainly more fluid and able to spread and fill any surface irregularities. Using hot glue, you can get away with a relatively thin stripe that will flatten upon closure and provide substantial bonding.

You can purchase double sided tape of many types, from thin, paper-only bonding up to heavy duty carpet bonding tape. The latter would work quite well in the cardboard arena, as it is designed to bond irregular, not-particularly-flat surfaces, but it's also going to be more expensive.