Yes, you can detect if a checkbox is checked on any event, using a Custom Javascript variable, value of which you then can use in the trigger, or in your tag. Or in a different variable, of course.
If you provide the html of your checkbox, we can give you code for the variable to return the checkbox state if you're not comfortable with JS.
//Upd
In your code, you have a few errors. First, no need to name the function in the CJS variable. Also, you write a bit too much code and rely on JQuery with no need. It can be solved simpler with pure JS like so:
function(){
return document.querySelector("input#add-subscription").checked;
}
Now, about the rest of what you're saying. Having this checkbox action. You have a wrong understanding. This is not a checkbox action. This code only gets the state of the checkbox whenever the value of this variable is being used.
So if you're gonna use the value of this variable when a person clicks on the form submission CTA, then this code will run on that click, assessing the value of the checkbox.
Your amazon example works. But you don't need to assume that the checkbox is checked by default. The default doesn't mean anything here. The state of the checkbox is assessed on the trigger where the value of the CJS var is used, that's it.
We're proceeding with your question: I'm confused about having the multiple criteria (checkbox is checked and user hits Checkout button) and if including both as triggers would work You're making another mistake here. They're not both triggers. One of them is a trigger indeed, which is a click trigger. The other, however, is a condition for the trigger to fire. You know how in the trigger, you click the checkbox that says "some events"? This fella:
Once you have that, you can add conditions. Like, only use this trigger when... when what? Anything really. In our case it would be when the checkbox is true.
Finally, for the checkout click, you don't typically need more than just the click trigger with the CSS selector criteria.
Anyhow, to be short, this is about how your trigger should look like:
