I'm building websites for the companies I work for with a few specific needs in mind. We're aiming to have invoice listing, owed balance,  and the option to pay bills online through their account. I also need to make getting the invoices into the accounts easy enough that completely computer illiterate individuals could do. My question is,  should I use wordpress,  Drupal, joomla... what system would be b easiest to implement the features I need? 
CMS selection advice
cms;customer service