There really is no way to buy 1000 miles for less than $37 and have them post to your account within 24 hours. I would just buy a block of 1000 and shell out the $37. You could also try making a purchase at shoponepass.com that would get you at least 104 miles, but the miles might not post within 24 hours of your purchase. And, the best deals often are for five miles per dollar, so you'd still have to spend at least $21 and would get fewer miles (if you purchased 1000 miles, you'd still have 896 left after your reward redemption).