Per Hour or Fixed Fee for early/late in-person check-in/check-out?

[Muslim4]

It is happening very often, now almost every guest has either one of those (early/late check-in/check-out). 

Since I have only in-person check-in/check-out, I always ask each guest before the confirmation. It is also clearly written in the House Rules and in the automatic online Welcome Message that extra fee is charged for each early/late hour. I've also added it to "Other Thins to Note". No, some guests don't even notice that! 

I often tend not to ask for extra fees if it's one or two hours (public transportation has limited working hours in our city, so I have no other choice, but to use expensive taxi to get to/from the unit), but when guests arrive or leave too early or late, it's becoming too difficult to get to/from the apartment and change my plans each time.   

I even decided to put it to the main listing description, despite there's limited space there. Hope it'll be more noticeable. I'm also thinking of switching to the fixed fee, because guest complain that it's too expensive and a few hours rate is often almost a nightly rate level. 

What I'm wondering is that do other hosts have clear rules and do you follow them? Do you set an hourly extra fee or a fixed fee? What do you usually do if a guest doesn't agree to pay because he/she thinks the price is unreasonable? If it's already after a booking is confirmed, do you refer to the House Rules?