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It works out around $10/12 per day per person. If you don't want to do in advance you could wait and deal with it onboard, either by using the envleopes they will provide and putting cash into them and givening them to the staff on the last night or agree to add the cost onto your cabin account. Which ever way you do it you will still have envelopes to hand out on the last night either with cash or a note saying prepaid, but the staff will know,but I have never found it makes the slightest bit difference as they all really do their best to make you feel welcome and make your cruise even more enjoyable. We have even although prepaid on the ship added cash to some envelopes before handing them over as some really did an excellent job looking after us.