Can I add my own markup to hotel rates before my customers see them?

Yes, in nearly every setup you can add your own markup to hotel rates before a customer sees them, and this is how most agencies earn their margin. The rate that arrives from the supplier is your cost, and what your customer sees is that cost plus whatever you have decided to add on top.

Most portals let you set this up in more than one way. You can apply a flat amount per booking or per night, or a percentage of the rate, and you can usually vary it by supplier, by city or country, by star rating, or by the type of customer, so a corporate account and a walk in customer need not be treated the same. Some setups also let you show a discount against your own selling price, which is handy when you want to run an offer without touching the margin underneath it.

What you cannot always control is how the supplier wants its price presented. Some contracts require taxes and local fees to be shown separately, and a few suppliers restrict what may be displayed beside their rate at all. Those rules come from the supplier and not from the technology, so what is possible for you depends on which suppliers you are connected to and on the plan you are running.

Before you go live, run a handful of test searches and compare what your portal shows against the cost you are actually being given, so you can be sure your markup rules are firing the way you expect on every supplier you use.