Yu-Pack is a parcel service offered by Japan Post which you can send from post offices, designated convenience stores or certain Yu-Pack(envelope) type can be dropped into the post box anywhere in Japan.
The designated convenience stores are Lawson. Ministop and Seicomart. Also you can track & trace your parcel and designate the time and date of delivery. Actually desinating the time and date of delivery is very common in Japan even for internet shopping. You can choose the delivery date and time upon ordering the products.
The postage of the Yu-Pack depends on the place you ship and place you want to send it. If you send from Tokyo to your friend in Osaka, the minimum postage is JPY 950. Also you can send your suitcase as Yu-Pack too. The postage for sending your large suitcase from hotel in Tokyo to hotel in Takayama is JPY 2370 It is convenient when you want to travel to Shirakawago before arriving at Takayama, where you overnight. Finding the locker is quite annoying these days and lockers can be already occupied when you find the lockers.
If you send by Yu-Pack, you do not have to pick up the suitcase again at the locker, not necessary to pull the suitcase from the station to your hotel which you might take bus or metro.