I drive ~50mi/day round trip, when I went through the F/I guy, he actually mentioned you can call BMW Financial and buy more miles (per mile is less than what you're quoting) as long as it's at least 6 months before the maturity date.
I'd give them a call and ask them. The reason I went this route is because I'll only pay for what I really actually use, rather than just buying 15/18k miles. They decrease the depreciation at the start. So it'd be a waste if I end up not using them.
2013 BMW 750i - Coded, Executive, Lighting, Cold Weather, M Sport, M 303 Wheels, 5% Tint, Conti DWS Tires, 3M Bra, Vinyl Black Gloss Roof Wrap, Exhaust Flap Mod.