You can use a service like Square using your iPhone or iPad using a card reader plugged into your audio jack. Using square you can actually enter the credit card number of the purchaser manually on your iPhone or iPad. If your purchaser is already a member of Square themselves you can even verify their identity from their member photo for additional security. There are other services that allow you to use your iPad or iPhone to process credit card transactions in real time from anywhere. So if business takes you out of the office you can take your credit card processing ability with you.