Guests see your upsells in their Guest Guide. The more often they open it, the more they order.
Messages: include the Guest Guide link in your booking confirmation and pre-arrival messages (see Smart Flow).
QR code in the property: go to Property, hover over the property, click the QR icon next to its name and click Download QR Code. Each listing has its own QR code, which opens that listing's Guest Guide. Print it and place it where guests look, like the welcome card, the fridge or the TV.
Tip: time your offers. A Show rule like "Time before Check-out: 1 day" puts late check-out in front of the guest the evening before they leave, exactly when they're thinking about it.