Well first off are you going there to buy panties for someone special or yourself? If for yourself, being a man you will no doubt get a sales gal coming over very quickly to help you. When she does, no doubt asking you who they are for and size, you can say they are for your special lady friend, but even better if you say they are for you. Then you will get all the help you need. Remember, no doubt they have sold lingerie to other men and admitting that they are for you will impress them with you honesty. In fact they most likely will mention other items, like a matching bra, etc. Remember, for that sale gal, a sale is a sale.
First, you have to know her panty size and panty style. Then, go to a regular lingerie store and explain to the store clerk (usually women) what you are looking for and for whom. Generally, they will be very helpful. If you are still embarrassed, might I suggest going online? Ebay has plenty of panties for sale or auction.
Well, usually you use cash or credit card. What do you mean how do you do it? If they are for your wife or girlfriend, make sure you know her size. If they are for you, check out a size chart in the women's clothing departments of an online store to find your size. Then, order online or get over your fear, go to the store, and pick them up. The cashier won't know who they are for. And if she does, who cares?
I totally agree with Lee! If they are for you, just tell them that! I have done that many times and I think I get help much more and faster then the women that are in there! The girls will also let you try things on and ask you if you need anything else to try and even bring you things that they think would be cute/match!