your options are to pay the amount or call the number on the notice for explanation. do it ASAP because you only have until the date in the notice to respond.
the failure to file penalty is a flat 5% per month or fraction of a month (4.5% if there is also a failure to pay penalty which is 1/2% per month or fraction of a month). so even filing and paying 1 day late invokes this 5% penalty. interest depends on when and how payment was made and may have nothing to do with date of acceptance.
