You pay for the sailaway cocktails with your cruise card. This is often the first thing that is bought on board. (Unless you buy a lanyard first to hang your cruise card on)
The sailaway cocktail can be bought in plain glasses $7.50, or the P&O plastic 'glass' for $10.00. You can then keep the souvenir glass.
And yes, they do have mocktails. They have a cocktail and a mocktail of the day, every day. These are printed in the daily newspaper which is left each night in your cabin.