It's a very common promotion for authorized ticket resellers (i.e. Undercover Tourist, Official Ticket Center, MapleLeaf, Orbitz, Expedia, etc...) to offer. Some offer it on base tickets others only offer it on PH and/or WPF&M tickets.
It is perfectly legitimate though usually doesn't save much ($10 a day at most; sometimes they'll still charge you tax for the fifth day).