[BizarroLand]

Why not put them in a plastic bag or Electrostatic bag and then tape them to cardboard? That seems like it would solve most of the problems cheaply without adding a lot of thickness to the envelope?

[mtzaldo]

He can also used printer paper. Put the card in the middle, and fold it to cover the plastic container.

[peddamat]

It really does seem like a small piece of tape inside the envelope would have solved the problem.