Go to the list of finance charges (which are part of your invoice list, you can filter your results by the right side of the page), and click on the finance charge you'd like to email. Then you'll have an 'email' link on the top right hand side of your page!
UPDATE: You can now create triggers that will fire when a finance charge is created, so you can just set a trigger to email them each time a finance charge is created - that will likely be more helpful than the original answer here!
