Reception already knows how each arrival booked, so the slip it hands over at check-in decides which booking window applies. That keeps the main flow exactly as described elsewhere on this page: no PMS integration, no channel manager feed, no guest account.
The advantage is an amenity rather than a discount. Distribution contracts generally govern the room rate you publish, not what you offer a guest once they arrive, which is why an earlier sunbed window sits beside a rate promise rather than inside it. Your own agreements are the ones that decide, so read them before you advertise it.
It also stays honest about what software can do. An earlier window changes who books first; it does not create more sunbeds. Hotels running short of capacity at peak should set the windows and reservation limits first, and treat the direct-booking advantage as a reason to book with you rather than as a way to sell the same lounger twice.