Yes it is. I always bring a partial, small roll of 3M shipping tape, the clear stuff, to add to the self-sealing boxes at the Correos. It helps. This is part of my standard 'repair' kit on Camino.
Leave the red plastic cutter, roll holder thingie home. Just bring the tape. They will have a scissor = tijera (tee-her-a).
The Correos boxes are very good, but IMHO, they could use a better sealing method. Not every Correos will have a roll of clear shipping tape available. They generally use clear cellophane type "scotch" tape, like on envelopes, and a bright yellow Correos branded sealing tape for larger items like boxes, and luggage.
I once shipped a nylon rucksack that was too large for their boxes. No problem...They just wrapped plenty of bright yellow "Correos" tape around it to secure all buckles, straps and handles.
Hope this helps.