She used dymo address labels in all of her correspondences because they were simple and discreet. She could order them herself through various places and pick them up at another location. There was no middleman to print the labels for her. She could do that herself because it was necessary.
It was the only way she could succeed on her mission. Otherwise, they would fine her and she would fail. And she couldn’t fail. She just couldn’t. Everyone’s lives depended on it.