On the 'net, you just have to keep an eye on your online credit card account. Unfortunately, there's no way to actually sign a receipt acknowledging the amount of the sale, and depending on the internet company, you could conceivably get screwed bigtime, and have to dispute the charge with your credit card company. Make sure to read through their terms!
In the outside world, you always have a receipt to both acknowledge the amount of the sale, and to sign, agreeing to the "terms" of the sale.
I would never agree to a credit card sale without first arriving at the amount of the sale. -kd5-