Creating a commercial website

I am on the process of creating my online web store.
The site is selling pin numbers. The pin numbers are stored in a text file, My problem is that after the customer complete the transaction, I want him to get in his receipt email the pin number, and at the same time, I want that pin number to be removed from my text file, so that the next customer will not get the same pin number as the previous one.
Any idea how can I do this?.