Leaving a tip on a credit card

[gilbert]

I took a job out of college here in North Carolina for a few months. Usually if I need money I'll go to the atm in my home town because there isn't a fee to take out money, but out here it's 2-3 dollars everywhere and since I'm lazy and cheap I don't go to the atm's, I just use my credit card.

The only time I eat out is at a china buffet or a barbecue place and I just put the check on my credit card, but since I don't have and cash with me I just leave the tip on the credit card. And I usually eat at both places a few times a week.

I've noticed at both places they don't pull the money out of the register and give it directly to the waitresses, but put it in an envelope with their name on it that's in the register. Should I tell the server I'm going to do this? I don't want them to think I'm stiffing them. Are servers used to tips going on credit cards nowadays and don't think they're getting stiffed? It just seems like an awkward situation.