Can I decide how much I add on top of the supplier price, and can it be different for flights and hotels?

Yes, you set your own markup, and you can keep it different for flights, hotels and the other services you sell. The software takes the price the supplier gives, adds whatever you have told it to add, and shows your customer only the final price. Your own cost stays on the back office side, where the customer never sees it.

Most portals let you choose whether the markup is a fixed amount or a percentage, and let you set it by service, by supplier, and often by route or by country as well. That means you can keep a thin markup on a busy sector where you are competing hard on price, and a wider one on a quiet sector where you are not. If you also sell through other agents, you can usually give each of them their own markup so your best agents get better rates.

There are limits that are worth knowing about. Some airline fares come with rules about what may be shown or charged on top of them, and a few suppliers give you a net rate that already has their own commission inside it, so what looks like your margin may not all be yours. Taxes and fees are normally kept apart from the markup, and whether your markup is added before or after them is something you should set on purpose rather than assume.

Once your portal is live, run a test search for each service you sell and check the final selling price against your supplier cost before you let your staff start booking, so you know the rules are doing exactly what you meant them to do.