I tried getting the discount as well last week. The CSR said there is a code associated with your phone number to validate you actually got the flier. Since she couldn't find a code for me when I called in (since I never really got the flier) she offered me $5 off/month as well.
So the $5 off seems to be the consolidation prize, which I'll take any day.
May be worth everyone's while to call in to try get the 15% discount, cause you may get lucky or get $5 off.
Of course, YMMV.