Web app ? introducing a pay as you use model. How to?
I have a very nice web app (HTML5, javascript,CSS) running on my website. 
It's free to use and people use it (as proved my google analytics)

I want to add some advanced functionality and get users who want to use the advanced functionality to pay for it. 

These days, what's the common way of achieving this ?

For instance I could send the user who has paid a key to a special to unleash the advanced functionality by entering in a text box. Or I could have a 'hidden' url which I would give just to that user. 

I can think of many ideas but I'd rather ask since I am sure there are standard ways of doing this. 
Thanks
Web app ? introducing a pay as you use model. How to?
===

I have a very nice web app (HTML5, javascript,CSS) running on my website. 
It's free to use and people use it (as proved my google analytics)

I want to add some advanced functionality and get users who want to use the advanced functionality to pay for it. 

These days, what's the common way of achieving this ?

For instance I could send the user who has paid a key to a special to unleash the advanced functionality by entering in a text box. Or I could have a 'hidden' url which I would give just to that user. 

I can think of many ideas but I'd rather ask since I am sure there are standard ways of doing this. 
Thanks