If you really want to maximize the 100 mile bonus, and you can afford to have some money tied up for a while, when you buy something, first buy (in the appropriate number of $100 increments) gift cards to the store, then use the gift cards to pay for the merchandise. That way, a $150 purchase becomes two $100 GC purchases, with $50 left on one card. If your purchase is only say $80, then pay for the merchandise, and also add on a $20 gift card (this is great at stores that allow to to specify the gift amount (ie. $18.75), since you simply get the total to $100 and not a penny more).

I'd recommend this strategy for stores that you shop at regularly (supermarkets, drug stores, etc.), where it won't take too long for you to use the remaining balance.

Jeff