Be careful when booking to make sure the flights are actually Song flights.

Song does not initiate PBI-BOS service until May 17. Right now, DL does have 757's flying the PBI-BOS route, but they are not Song planes. They are standard DL 757's.

When booking Song flights on Delta.com make sure the flight actually says, song 2XXX. Don't assume just because there is a 757 on the route that it is Song.