This is governeed by state law - this was great income for my previous employer but, in Tx the only charge allowed is the accrued interest.
IMO - if your letter describes the fact that this is a fee for a service (extending the pmt / skip-a-pmt) then more detail stating the amount is not going to Prin or Int is not necessary.
Also, this is an event that is occuring later (like a late fee) & not anticipated at loan origination, so don't let someone tell you that you need to disclose a revised APR or you can't do it because of Reg Z.