Custom field problem

[kassandra77]

Hi I have a problem with the custom field.
I would like the first item in the list of default values ​​to be set without adding extra.
I chose the option "Automatically add an empty option" but on the site even without choosing a value from the drop-down menu it automatically adds the extra to the base cost. Some idea?

test site http://www.tuttoriposo.it/index.php/divani-letto/beatles-detail
joomla v 3.9.3
vm v. 3.4.2