The terms say that a more expensive domain extension costs a surcharge on top of your plan

Common extensions such as .nl, .com and .dev are included in your plan. A more expensive one, such as .app or .shop, has a surcharge per month on top of your plan when we register it as the domain in your plan; the domains page lists each surcharge. Article 20 of the terms now says so, in version 9, and the help articles no longer list .app as included; it costs € 0.50 a month extra. Nothing changes for a domain you already have.