It's a surprise. It's a gift to be opened. Your customer already has your card. Now she can give this one to a friend. There have been many discussions about enclosing business cards.
Some people think it's pushy.
It signals that you're trying to solicit more business. You appreciate your customer's patronage, and you would also like to assist anyone else she may know who needs your expertise. You can't service the public if they don't know you exist. I would recommend that you do this later in the selling relationship, immediately before she exits - not when she first enters. That's what every other clerk in town is doing.