From reading the responses on this thread I think many do not know what a prepaid tip is. It is not the automatic tip that is added to your sail & sign account. It paid at the time of booking or at time of final payment or some time between. (hence prepaid)
For norfork-carl on ships that have the automatic tipping they do not provide envelopes for additions to gratuities. On one ship I was on they did provide an envelope for the Maitre d' as they are not included in the gratuities package.