Shopping Cart to book extras - for guests that booked room via another online booking system

I realize solidres is for booking hotel rooms.

that said, i'd love the options for my guests to book transfers, tours, spa, etc ... (extras) without having to book rooms. Guests may have already booked rooms online with another booking system (gds) and I want the ability to upsell.

this feature is especially important for spa & gift cards. we are a resort BUT the spa is also open to day guests.

Does this feature exist already?

Thank you for your time & attention.

MN