- if you're sending your luggage to the airport, check the deadline for sending, depending where you are sending from it could be 2-3 days before your flight
- hotels I paid at were cash only for this service (one of the few things I paid in cash, but likely depends on the hotel)
- some hotels did not take payments for sending, only supporting cash on delivery, so if you're sending to another hotel you should check if they will accept it
Sending from a convenience store is another option if those two last points are problems.
Another thing I thought of, the two times I needed it, the hotels sold cardboard boxes (a few hundred yen) and provided packing tape for free.