Probably springtails, they’re attracted to damp so probably living in the paper. They’re harmless. Bedbugs wouldn’t live there. They like warm claustrophobic environments and want to be near people.

Smart to be cautious though

Put a layer of snail pellets on the base of the letterbox, then cover with a sheet of baking paper cut to size. Should stop any further snail invasions. And keep your mail nice and dry.