Can my customer ask for something special, like a late check in or a high floor, and will the hotel really see it?

Yes, you can give your customer a box to type a special request, and it will be passed along with the booking, but nobody can promise the hotel will act on it. The request travels from your website to your supplier, and from the supplier to the hotel, where it sits on the reservation as a note for the front desk to read when the guest arrives.

What you can send depends on the supplier you are connected to. Some accept a line of free text in English and pass it on as written. Others only accept a short set of standard requests, such as a non smoking room, twin beds instead of a double, a room on a high floor, or a late arrival, and anything typed outside that list is quietly dropped. It is worth finding out which of the two your supplier does before you put a big empty comment box on your booking page.

Be honest with your customer about what happens next. A special request is never a guarantee. The hotel decides on the day, based on what rooms are free when the guest checks in, and some things, like an early check in or a connecting room, may be refused or may carry a charge the guest pays at the hotel. Wording it as a request rather than a confirmed extra saves a lot of difficult conversations later.

Make one test booking with a request on it, check that the request appears on the confirmation your supplier sends back, and ask your supplier which request types they actually forward.