Millenium and interent package

We are going on this ship next February, Singapore to Hong Kong, our first with Celebrity. We will want the 24/7 internet package, do we just buy once onboard or do we need to buy beforehand, I know when we buy we need to pay. Have already booked 3 different trips to make sure we get them but had to pay upfront.

I remember years ago on Princess booking trips beforehand but they were added to your shipboard account.