Where can I find a service that will hold a letter/package I send for some time then mail it back to me?

I want to force myself to take a break from using a device that I sometimes find it difficult to resist. I want to lock it up in a home safe, and then make the key unavailable to me for about a month or so. I'm looking for a reliable service that I could mail the key to and have them mail it back to me a month later. The service needs to be relatively inexpensive (say $20 or so max), reliable (not going to lose it or send back at the wrong time), and discrete (I don't want to have to explain why I'm doing this).