Will my portal warn my customer about city taxes or resort fees they have to pay at the hotel itself?

Yes, when the supplier tells us about them, and most of the larger ones do. Some countries charge a tourist tax per person per night, and some resorts add a daily fee for things like the pool, the gym or the internet. These are collected by the hotel at check in rather than taken when the booking is made, so they sit outside the amount your customer pays you. Where the supplier sends that detail, the portal shows it on the room and again before payment, worded as a charge payable at the hotel.

The reason we cannot promise it every time is that this information comes from the supplier feed, not from us. A well maintained property will list the tax clearly, the currency it is charged in and who is exempt. A smaller one may say nothing at all, or may describe it loosely in the notes rather than as a figure the portal can display on its own line. When nothing is sent, the portal has nothing to show, and your customer meets the charge at the desk.

This is worth taking seriously because a surprise at check in is the sort of thing that gets blamed on the agency rather than the hotel. If you sell a lot in destinations that apply a tourist tax, it is sensible to add a short line of your own to your booking terms and your confirmation email saying that local taxes and fees may be payable directly to the property.

Run a test search for a destination you know charges a tourist tax and see how clearly it appears in the room details, then decide whether you want your own wording added on top of it.