It's not advertised. I saw it on an old post here and then googled Disney Embarkation Services and found their number which is 800-395-9374. Their hours are 7:00 AM-8:00 PM (EST). You can call up to 48 hours ahead to schedule the luggage transfer. They will check their schedule to make sure they have more than one pick up at the resort. I was especially interested because we took a one-way rental from OKW to the port. We did not want to have to separate and have DH drop us off and then return the rental and try to meet up with us.
I used them over a week ago and it worked flawlessly. I called on a Thursday and they said they will service OKW. On Saturday, we left our bags with the
DCL tags in our room and checked out. They work with the resort's bell services and scheduled to pick up our bags between 8 AM to 10 AM. I stopped by bell services and the CM said he just called for someone to pick them up. We drove to the port in a one-way rental and our party of 3 was able to easily take the shuttle to the port altogether since we didn't have luggage to worry about.
When I checked our onboard account on the TV under guest services, I saw the $25 luggage transfer was already charged. Our bags showed up after muster drill.
Hope this helps.