The first step is to define the requirements.
An idea is not the same as a set of requirements. A set of requirements is not the same as functioning code. But, there is a progression from one to the next.
You have the idea, and at least a vague concept of the requirements. Now, it's time to flesh out the requirements. Think about this from the perspective of trying to hire someone to write the code for you.
What does the Arduino need to be able to do? Does it only receive text messages (Sell me A3), or does it send them, too (Item A3 dispensed).
How will the text message sent to the machine result in a bill being sent to the phone owner? My mobile phone doesn't have any money in it. I can't do anything to it to shake any money loose.
Certainly the Arduino, with appropriate shield, can receive text messages. It can, with the appropriate hardware, communicate with paypal to pry money out of the texter's account.
Whether you can write all the code to make this all happen, though, isn't clear.