Solution #2 - use a global flag

The solution most people would probably think of first, is using some global variable to record whether the user has already clicked on the button. You'd define a flag named something like clicked, initialized with false. When the user clicks on the button, if clicked was false, you change it to true, and execute the function; otherwise, you don't do anything at all:

let clicked = false;
.
.
.
function billTheUser(some, sales, data) {
    if (!clicked) {
        clicked = true;
        window.alert("Billing the user...");
        // actually bill the user
    }
}

For more good reasons NOT to use global variables,
read http://wiki.c2.com/?GlobalVariablesAreBad.

This obviously works, but it has several problems that must be addressed:

• You are using a global variable, and you could change its value by accident. Global variables aren't a good idea, neither in JS nor in other languages.
• You must also remember to re-initialize it to false when the user starts buying again. If you don't, the user won't be able to do a second buy, because paying will have become impossible.
• You will have difficulties testing this code, because it depends on external things (that is, the clicked variable).

So, this isn't a very good solution... let's keep thinking!