I visited Tei-Park (Communications museum) last Sunday. Regular admission fee is 100 yen ($1.02 €0.78) for adults. Fortunately last Sunday was in Science & Technology week and the museum was open without any charge. The following photo shows "Today is free of charge." It also says "There is no charge for student on holidays."
There are many exhibited objects related postal service and telecommunication in the museum. The collection of stamps is interesting. You can see stamps from all over the world, for instance, a stamp of Dutch Curacao. For details, please see the website of Tei-Park.