You can join Mustang Club of America, for $50, and get X-Plan pricing.
I usually just ask for the invoice and agree to pay $200 over invoice - rebates.
Invoice on my car was $38154.52 - $500 (switch rebate) + $200
I paid $37854 for my 2015 Premium PP w/Nav. The dealer has 3% hold back on invoice and gets survey money from ford. No need to ever pay near MSRP.
2016 Mustang GT Roush TVS 2300, MBRP race catback 611 rwhp
2015 Focus ST Race Red gtx2971r w/aux fuel (447 whp)
2015 Mustang GT PP (Stock)
2015 Fiesta ST (305 whp) (gtx2860r)
2015 Mustang GT Premium PP (410 whp)