multiple discounts / coupons / vouchers

at the moment EMPro allows single discount or coupon per event only – https://wp-events-plugin.com/documentation/coupons/

Do you know about a way / how to extend, that I can manipulate the price – but checking some checkboxes?

Can you explain a bit more about what you want to do?

I would like to add some custom fields to my form – when I check those fields the user gets a discount – so the overall price will be changed.

That would require quite a bit of custom coding. One way to do it would be to use jQuery to manipulate the form as needed.