Why Is ‘I’ll Have a Flat White’ a Natural Way to Order?

In an Australian café, “I’ll have a flat white, please” means that this is the drink you are choosing. Have does not mean permanent possession here; it is a conventional, polite way to place an order.

If the server asks, “What can I get you?”, you can reply, “I’ll have a flat white, please.” “Can I have a flat white, please?” is equally natural. “I want a flat white” is grammatical, but in an ordinary service exchange it can sound unnecessarily direct.

I’ll have is not uniquely Australian; it works in British and American English too. The local flavour comes more from the flat white and the relaxed café setting. Once the order is complete and the coffee is already with you, do not use I’ll have to describe possession; say, “I’ve got my coffee.”

https://learnenglish.britishcouncil.org/free-resources/listening/a1/ordering-cafe
https://dictionary.cambridge.org/grammar/british-grammar/also-as-well-or-too