Check your sight glass, if the oil level is fine the chances are they just stick 3.5 on the bill because most common inline 4s take that, they're just being lazy with the billing. It does look like you paid for a litre you didn't get, ask for it to be returned.
I remember years ago new plugs and points were on the bill of a service for one of my cars, it had electronic ignition, no points (for the youngsters). I queried it and the bloke said 'oh yeah, sorry, we put that on all the bills' and knocked it off the cost. :o
And folk wonder why I do all my own servicing. Sometimes it's a chore, I'm not looking forward to the Glees valve clearance, two heads equals twice the work, but it's the only way I'll know it's been done and done properly.