Say you want to charge a $5 late fee. Do you charge this to each invoice that is unpaid, or just to the last one sent out? We invoice once a month and it isn't uncommon for someone to have two invoices out at one time. I even have one that has 3 invoices because they were canceled the first week of the 3rd month. Should I be adding $5 a piece for a total of $15 each month.?