Can my customers pay for the hotel when they arrive instead of paying me when they book?

Yes, in many cases they can, but it depends on the rate the supplier is offering. Hotel rates come through the API marked as either prepaid, where the money is collected at the time of booking, or pay at hotel, where your customer settles the bill at reception. Both kinds can turn up in the same search for the same property, at different prices.

Your portal can show which type each rate is before anything is confirmed, so your staff know what they are selling. If you would rather keep it simple, you can usually choose to show only one kind, so your agents are not picking between the two on every booking. Keep in mind that a pay at hotel rate still normally needs a card to hold the room, even though no money moves at the time of booking.

The honest limit is that you do not decide which rates exist. The supplier and the hotel set that, and some properties, and some countries, offer prepaid rates only. Your commission also works differently on a pay at hotel booking, because the money never passes through your hands, so ask each supplier how and when they pay you on those.

Before you go live, run a few searches in the cities you actually sell and see what mix of rates comes back, then agree with your suppliers how you want the money to reach you.