Declined Credit Card Fee
Every other hosting related software has this, including AWBS which has not been updated for years, its hard to believe that WHMCS does not have this feature at all.
Basically everytime a credit card charge attempt happens, we pay a $0.30 fee to the processor. If its declined, there should be a way to have WHMCS automatically add a "Declined Credit Card Fee" to the invoice ($0.50 in our case, but should be settable in the admin). This should happen every single time. So if a credit card is declined 3 days in a row, there would be 3 $0.50 charges added to this invoice.
This also causes clients to update their credit card faster, but as well generates some micro-income for the business. With AWBS, this would generate $1000s of dollar of extra income each year.
We are working on a hook to implement this feature, but really should be built in.