Depends on how low you want to fly under the radar. If the feds/dea really looked into it, I don't think even a prepaid gift card would do much to befuddle or stop them. It would be a bleep on their radar, and if I were a fed, the first thing i would in investigating that "bleep", is check electricity consumption. You have to put a name and address associated to the card and then the order. Only way around that might be using a false name, but then the paper trail still leads back to your address. Unless you can have it sent to a neighbors house or something. Seems like a lot of unnecessary trouble. Money order is maybe a more covert option, using a false name again. Its a tough call. Go through the trouble of buying and activating a prepaid card, and then even more trouble finding a place to ship it. Or wait the extra time for a money order to reach the supplier. Money order seems better. Whose to say the feds cant trace a money order though? The usa government has a lot of power, and when they want something, they don't let things like "rights" , "laws" or privacy stop them.
I had a roommate a few years back that bought seeds and grow equipment online with his credit card and nothing ever happened to him. He probly still grows. I moved out of the house though, becaues of that. Not only did i not want to live in an illegal grow house, but didnt want to live in a grow house full of idiot party animals, that and we lived next to a cop. I myself have bought seeds online with credit card, but I threw them. Too chicken shit to grow or even have that kind of incriminating evidence in my house. Also bought a grow light onine, but I've only used it once to grow tomatoes. Wont use it again til I can legally grow weed. (Too much trouble/time for tomatoes I could just buy at the store.)
All in all, its a risk either way, and once you make a bleep on the radar or are entered into "the big machine", ur sticking ur head on the chopping block. But, unless you are dumb, conspicuous, or have a major grow op, the few hundred dea agents employed probly will not waste their timetaking a 'chop' on you.