By default, clients can add or remove add-ons when reviewing a proposal. If you want to restrict the proposal to exactly what you included, you can disable this.
Disabling Add-On Changes
Open the proposal.
Click Manage Booking.
Scroll to "Should the customer be able to add and remove add-ons before accepting?"
Select No.
Save.
This setting is per-proposal. You can allow add-on changes on some proposals and restrict them on others.