This is one of the benefits that MP has over many (most) other carriers. They charge origian to destination rather than by segment.

I am flying BUR-ORD-MSP/ MSP-ORD-SFO-BUR.

Direct BUR-MSP is 1522 miles, thus only 3 coupons (as there is a 50 mile grace). But on that return, I am actually logging over 2500 flown miles. But I will still be charged for only 1500!