The $82.50 to buy 3000 miles is a pretty meaningless metric, in my opinion.

First, with any small mileage purchase, the one time fee adds a significant bite. That's 2.75c/mile, and at the higher end, it's down to 2.5c/mile. But there's also a $30 processing charge, and a bonus too.

Second, the cost to buy miles directly should only be a guidance for the top range of what you'd effectively pay. If you can get miles at this price, never pay more embedded in another transaction. If you want the miles, just do the other one in a less expensive way and buy the miles directly. Rarely, though, is this the case.

Finally, while I do think 3000 miles are worth $20, I think you'll get even more out of the deal by renting from Avis rather than Enterprise. You're more likely to find an on-airport location, plus the customer service and the quality of the cars are just worlds apart between the two companies.