I've sent in feedback on this, but want to comment on the thread for others to see.

Using the automatic interest has 2 significant drawbacks:

1.  There is no way to opt individual clients out of the process.  For example, if we have customers that we will never want to charge interest on, that is not an option.  All or none.

2.  There is no way to extend the grace period beyond 30 days.  We do not want to charge interest until the invoice is 90 days (our large customers complex processes mean we are paid at around 60 days, not 30, so interest would not be appropriate at 60 days).  The only workaround is to change our due dates.

The only other option for us is to do an invoice each month for interest, which means not only manually creating an invoice, but also doing manual calculations rather than having the percentage calculated.  This is extremely tedious and open to error.