Can I add my own commission on top of the hotel prices that come from a hotel booking API?

Yes, in most cases you can add your own commission on top of the prices that come from a hotel booking API. The supplier sends your system a net rate, which is the amount you owe them for the room, and your portal decides what the customer actually sees on the search results. The difference between those two amounts is your margin, and it stays with you when the booking is paid.

How you set that margin is usually your decision. Most travel agency systems let you apply a markup as a percentage or as a fixed amount, and let you vary it by supplier, by country, by star rating or by an individual property, so you can be sharper on the hotels you sell most often. Many setups also let you give one markup to your walk in customers and a different one to agents or corporate accounts logging in to your portal.

There are limits worth knowing about before you plan around this. A few suppliers and a few hotel chains work on a fixed selling rate and do not allow you to change what the guest is shown, and some agreements set a ceiling on how much you may add. Whether local taxes and hotel fees sit inside or outside your markup also depends on the supplier and on the rules of the country you are selling into, so the same percentage will not always behave the same way on every booking.

The sensible next step is to ask whoever supplies your hotel content what their rules on markup are, then set a modest default in your system and run a few test searches so you can see the customer facing price for yourself before you go live.