Will a hotel booking API tell me about extra charges my customer has to pay at the hotel itself?

Yes, in most cases, although how much detail you get depends on the supplier sitting behind the booking.

Plenty of hotels charge things that are not part of the room rate, such as a city or tourist tax, a resort fee, a cleaning charge or parking. A hotel booking API will usually pass these through as separate items alongside the room rate, and it will normally say whether each one is taken at the time of booking or paid straight to the property at check in. When the supplier sends that detail through, your portal can show it on the room result and again on the checkout page, so your customer sees the whole picture before they pay rather than getting a surprise at the front desk.

There is a limit worth knowing about. Not every supplier sends this information for every property, and some send it only as a line of plain text rather than as a proper amount your portal can add up. Where nothing usable comes through, your portal shows the wording exactly as the hotel wrote it and make clear that local charges may apply on arrival. That is why two hotels on the same results page can look quite different, one with a tidy breakdown of charges and the other with only a short note.

Go through a few properties on your own portal, in different countries, and compare what your checkout page shows against what the hotel publishes on its own site. If the detail looks thin for a supplier you sell a lot of, raise it with your account contact, because it is often a matter of switching on the fuller rate detail on that connection rather than changing anything on your website.