Well you will call me crazy...
For my last cruise I pre-paid my tips but I also gave them a little extra $$$ tip for each one at the end of the trip for their service.
Is not much but it's always good to let them know they make an excellent job...and that's the main reason I love cruising.
You are not crazy. In fact I do the same thing. They get the envelope from me with the voucher for the gratuities I added to my account - plus some extra cash depending on how good the service was.