En ventanilla

Hola Peter 

There are some set expressions in Spanish where the article is omitted when we refer to something that is seen as "general". This is the case of "en ventanilla" meaning "by going to the desk" [where you generally pay]. It's not referring to any specific desk/window with a number or anything. It is referring to paying by going to one of these desks. 

If we were saying to someone for example, "you need to go to desk number 4" more specifically, then that'd trigger the use of the article as we are not referring to simply "paying at the kiosk/desk" but paying at one specific one. So we'd say:

Another typical phrase where we drop the article, as it is seen as a set expression talking in general, is for example when someone asks you if you're paying by card. You'll hear:

There is no article here "con la tarjeta/con una tarjeta" (???) because they're referring to the general transaction made by paying with a card (whichever card it is you're paying, that's not relevant)

I hope this clarified. 

Saludos

Inma