As a relative newbie, I am aware that the donation is to be placed in an unsealed envelope, etc., and left discretely on the nightstand/desk upon arriving at a provider's room.  Also, that no discussion of rates relative to services is appropriate.  That being said, what is the proper etiquette should I decide the session deserved a tip, or, for instance, if the provider and I mutually agree to extend the session.  The original envelope has disappeared.  What do I do with the additional cash??