selling extra "experiences" or items

I'd like to offer my guests the option to buy extra items (t-shirt or coffee mug, for example) and/or an extra experience (the romance basket, for ex) during their booking process. Is it possible to "upsell" items during their booking?

Yep, you can do this with an optional surcharge that the guest can choose during the reservation process. More on that:

https://www.ownerrez.com/blog/add-ons-are-here--also-known-as-optional-surcharges
https://www.ownerrez.com/blog/enhanced-surcharges-discounts-and-add-ons

If they want something during their stay, there's no guest form for that currently. Instead, you'd edit the charges of the booking and then run a payment on the booking.

Got it! Thanks!