Hey Javier,
Thanks for reaching out!
This is unfortunately not possible within our own plugins by default. Only our “Community Tickets” add-on has this feature—is that the plugin you’re describing?
I assume “no”, and I assume you are referring to Event Tickets Plus.
There are ways to add fees like you describe, but they unfortunately are done via WooCommerce itself, not our plugins, so for example check out this page:
• https://docs.woothemes.com/document/add-a-surcharge-to-cart-and-checkout-uses-fees-api/
I searched a bit and found this free plugin that does something like that, too:
• https://wordpress.org/plugins/woocommerce-extra-fee-option/
And so on—check out some of these options and others, and post a question to the WooCommerce support folks too if necessary, and see if you think you can make things work as you hope….
I hope this helps!
— George