I searched for this and couldn't find it. However I do know this is somewhat of a repost, however this will go into more detail.
So basically, this is illegal, and can get you arrested, but ive done it numerous times and never been caught.
Step 1: Go to your local Walmart or Target, or any other department store that sells WoW gametime and/or CD-keys, but ALSO sells clothing.
Step 2: Take a shopping cart. First go to the clothing section, and act like you are looking for clothing. Take one or two shirts and throw them in your shopping cart.
Step 3: Now you head over to the section where they sell WoW gametime, or CD-Keys, or whatever. Take your box with the gamecard in it, and place it ontop of the shirts.
Step 4: Go to the dressing room, as if you were going to try on the clothing in your cart. Stop your cart right outside the dressing room. As you pick up the shirts in your cart, make sure you pick up the gamecard box with it. So basically the gamecard box is kind of wrapped in the clothing. Please do not epiclly phail and make this look obvious. Just so the cameras can't see the box inside the shirts.
Step 5: Take the shirts inside the dressing room, and close the door. There are NO cameras in the dressing rooms of these stores!
Step 6: Since there are no cameras, you don't have to worry about them seeing this step. Rip open the box. Scratch off the code part so you can actually see the code. Write the code down on your arm or on a piece of paper, something. Make sure when you right it, you right it CAREFULLY and EXACTLY right.
Step 7: This step is optional, however it makes you seem a lot less obvious. Take the shirts back out of the dressing room and put it in the cart. Take the cart back to the clothing section, and place the shirts back as if they didn't fit. Now go back to the gaming section where you got your game card. Act as if you are putting something back, although you aren't really. Now take the cart back to where it was, and leave the store.
I know that this was posted before somewhere, but I feel I went into way more detail. I couldn't find the post when searching for it, but I do remember that the post was very general, didn't explain anything really good, and was jokingly written.
I have actually used this a lot of times, and it has never failed. I have thought about just taking the card out of the store to ensure that I get the right code, as one time I wrote down the code wrong, but I am too scared. I don't know if there is some security thing on the actual card, or what. So anyway, like I said before, USE AT YOUR OWN RISK!!!!!!
If you can find the original post, feel free to post a comment with the link. However, please don't come here saying ZOMG ITS TEH REPOST!!!! Because honestly, I know this already, and have stated so about 5 times. However, I wrote this to further explain step by step EXACTLY how to pull this off the safest way possible.
If you can find the original post, and it does go into as much detail as I did, then go ahead and flame that I reposted. However, from what I remember, it had about a paragraph saying something like "go to the store. take the gamecard and try to hide it from the cameras and rip it open then take card and walk out of the store. if they see you just run as fast as you can, once you hit parking lot they will stop chasing." However I found that this method is completely idiotic unless you want to go to jail and take the chances of dropping the soap!