Can my customers book business class or premium economy on my portal, or only economy?

Yes, your portal can sell the higher cabins as well as economy, and the customer normally picks the cabin before the search runs.

The cabin choice is passed to the supplier along with the route and the dates, and what comes back is whatever that airline has left for sale in that cabin on that day. Business class is the most reliable of the upper cabins, because nearly every airline files it the same way. Premium economy is the one to watch, since airlines describe it in different ways, and a search for it can quietly return ordinary economy seats from a carrier that does not really sell a separate premium cabin. First class appears only on the small number of airlines that still offer it.

How complete the results are also depends on the supplier feed behind your portal. Some feeds carry the full range of cabins for the airlines they cover, others are set up mainly for economy content and will return very little when a higher cabin is asked for. If your business leans towards corporate travel, this is worth checking before you go live, rather than finding out when a client asks for a business class quote.

Run a test search in each cabin on the routes your customers use most, look closely at what comes back for premium economy in particular, and ask your supplier which cabins they support for the airlines that matter to you.