On all Celebrity ships you may IF YOU WISH have your gratuities debitted to your on board account. You have to sign for this on a seperate form, which is then handed in to Reception.
You will receive a receipt for each person and an envelope, you put the receipt in the envelope and gice it on the last night as usual. You could for instance give the suggested amoun on your account and then put into the envelope a little extra together with the receipt.
If you do not fill out the form, you will receive envelopes only as in the past.
I have used whichever method suits me, usually using the on board accoutn mtethod if I am on a long cruise where carrying money in cash would be cumbersome.