I just read the Mint's blog and apparently they also read the newspapers which recently detailed how people are getting tons and tons of free airline miles by using the direct ship program. It cost nothing to get the direct ship coins and you could also order as many as you like. So, for instance, I order $10,000 worth, receive them and get 10,000 airline miles. I then drop them all off at different banks, pay off my $10,000 credit card, keep the miles and start all over again. There are some innovative folks out there. To put a stop to this the mint now has a limit of 20 boxes going to one household. They also will process your credit card as a cash advance which does not earn miles. I guess this is a way to stop people's innovation but I don't care for it as I don't earn miles on the one box I usually order yearly AND it cost me 5 percentage points on my credit card i.e., if my interest rate is 14% my cash advance rate is 19%.
Probably cost you more than that since if you paid the bill off in full there would be 0% interest as a charged item, but the 19% cash advance starts charging interest from the time of the advance even if you pay off the bill on the due date.
I would say all credit cards as those accepting credit cards don't know if they are or are not mileage cards.
It appears to be "scout's honor" still but that proviso that states that you agree not to engage in that unethical practice may have some teeth (I bet a lawyer wrote that).