Will the hotel price I see through the API include all the taxes and fees, or will my customer be charged more at the hotel?

Mostly yes, the price returned by a hotel booking API includes the taxes and fees that the supplier collects from you at the time of booking, but some charges are only payable by the guest at the property and those are shown separately rather than added to the total.

The reason for the split is that hotels handle money in two different ways. Room rates and the taxes tied to them are usually settled through the supplier, so they appear in the figure you quote and you can treat that as the amount your customer owes you. Local charges such as a city or tourism tax, a resort fee, a parking charge or a refundable damage deposit are collected by the hotel at check in or check out, because the supplier never touches that money. A well built API passes these through as a list of charges payable at the property, with the amount or the way it is calculated, so you can read them before you confirm anything.

How complete that information is does depend on the supplier and on the property. Most of the larger hotel content feeds describe mandatory local charges clearly, but smaller independent properties sometimes give vague wording, and a tax rate set by a local authority can change at short notice. Because of that, we treat these fields as information from the supplier rather than a guarantee from us, and we show them to you exactly as they arrive instead of guessing at a total.

When you are testing your hotel connection, book a property in a city that you know charges a tourism tax and check what the response tells you about charges payable at the hotel, then make sure your quote and voucher repeat that wording to the guest so there is no surprise at the desk.