Form the user's point of view, they will see a link on checkout pages all over the web that says something like: Sign up for "keep the change+1". That takes them to a sign up page where they can select thier charity and enter thier preference for participating in the program. OR the charity websites can have sign up pages, with links to all participating online stores.  Once they are signed up, whenver they go to checkout from a participating vender, there will be a checkbox marked:  Keep the change +1: Support the Musella Foundation (or the charity they select).  IF the user checks that box, $1 + the rounded up portion of the amount is added to the final bill.