Does anyone know where I can find the explosive ink used by department stores to prevent clothes being stolen?

My mom keeps getting her packages stolen from her doorstep and I want to try to find out who took them. She lives in an apartment complex that locks the very outside door, so we know it's someone who lives in the building. I figure I will send a package that when opened and picked up will explode ink on the person's hands. This way we will see the ink on the person's hands and know it was them! I just don't know where to get the stuff. Or if you have any other ideas on how to catch the guy (or girl) let me know! I've already considered a hidden camera, but that seems expensive. Thanks!