Do I need to pay separately for every sex act?

You agree in advance on what you want from the prostitute. She will tell you what she will and won’t do and how much it will cost.
Vaginal sex and blow jobs are usually standard. If you want to do anything else, you will often have to pay more. The extra price depends on what you ask for.
The price also depends on the prostitute and her boundaries and what she wants. For example, some prostitutes won’t kiss a client and others aren’t bothered by it, provided you pay more.