Can I decide how much I add on top of the supplier fare before my customer sees the price?

Yes, in most cases you decide what is added on top of the supplier fare, and your customer only sees the final price.

A white label portal is built to be sold under your own name, so the commercial side is usually left in your hands. You can normally set a markup as a flat amount or as a percentage, and apply it differently to flights, hotels and other products. Many setups also let you vary it by supplier, by route or region, or by the type of customer, so a corporate account can be treated differently from a walk in traveller. The customer sees one total price with the usual taxes and fees, never your own cost.

There are honest limits, and it is fair to know them before you plan your margins. Some airline and hotel content comes with supplier rules about how a fare may be displayed or sold, and in those cases the room you have to add on top is smaller, or the amount has to sit alongside the fare as a service fee rather than inside it. Payment gateway charges are their own line and are not something you can hide inside a markup. How detailed your rules can be also depends on the plan you are on, since finer control by route, cabin or staff group is not always part of a basic setup.

The sensible next step is to write down the rules you actually want, such as what you add on flights, what you add on hotels and whether any group of customers is treated differently, then check each one against the supplier agreements you already hold before you go live.