Can I offer to reserve additional resources only after booking the main resource?

Create the resource group.

Put in it the resources you want to offer when booking the main resource.

Open the widget. Below For adding to an order use specify the created group. Save.

Add a hint on the reservation and order form for additional services. See here for how to do this.

You can also change the Add more to order button to your text. To make it clear to customers. See here for how to do this.

You can place the Add more to order button at the bottom of the cart. To do this, go to the Appearance tab. Set the check box Use custom css-styles and paste the following CSS:

See an example in action.

Get this result: