For an upcoming event, we are requiring a security deposit that everyone must pay. How do I add that to the price set so that it's always there and the user does not have to do anything with it and cannot remove it?
Try to add price_set_field for the same in the price_set.
so, it remain always also make it as radio (mandate)
I guess this helps !!!
You can add price field to a price set used for an event. You can navigate to CiviCRM >> Administer >> CiviEvents >> Manage Price Sets.