Yes it is a scam,that money order will turn out to be worthless. If you have deposited it, notify your bank immediately, do not withdraw any money, lest you will be responsible for that money and the fees involved.
If you have not deposited it, take it to the post office along with the envelope it came in and have them forward it to the postal inspectors.
All these ads for puppies are a scam. Perfect reason for you to get your new next best friend at the shelter. You get to meet it and take it home the same day for a lot less money. And just think how great you will feel knowing that you helped out an animal in need of a home.