anywhere that takes Credit Card is not considered cash advancement. the only time this qualifies is if you use the card in an ATM to get cash.
and because of this, Post offices do not allow you to use credit card to purchase them MOs.
same reason how you cannot pay off your car payment or mortgage every month with your credit card. although I do find it weird that when I bought my car, they did allow me to use my credit card as down payment. I guess they don't want me collecting points on something that is considered interest? (since down payment is all going towards the principal amount)