Hi @Ruby90
If you don't mind me saying so, you sound like a really reasonable, responsible guest who really cares about using the platform appropriately and take the time to educate yourself and learn more.
I think you'll make an wonderful guest for future trips.
I think the term "cleaning fee" is really unfortunate because naturally, it does set up a certain set of expectations in a guest. I bet if you had payed exactly the total price for your stay and there was no "cleaning fee" you'd have felt quite differently. (I personally dont' charge a cleaning fee at all for this exact reason, I just add an amount to the total rental price adn then add a discount to stays over 3 days to offset the price hike)
Before I was a host, I know I would have felt very similarly to you. Since you seem so receptive, i thought I'd just offer the hosts perspective. I own a large home ( 4 bedroooms) and I'd say on average it takes me about 8 hours to clean, wash linens, change beds etc etc. This is ( sort of) what the cleaning fee is MEANT to cover - the very basic preparation to get ready for the next guest. Many hosts dramatically undercharge especially if they're doing the cleaning themselves.
If a guest leaves a stain on something - for me it's usually heavy makeup stains on towels - that can take me up to 1 -2 extra hours of soaking and washing to get the stain out. Its exasperating. And expensive.
I agree with @Sarah977, a stain on a sofa could be extremely difficult to remove - particularly if it had soaked in and dried and may well need an expensive professional clean. ( I do wonder though why the host hasn't scotchguarded a fabric sofa - I mean it's hardly bizarre that a guest would take a coffee to the sofa. )
I CAN see why the host would want money to clean the extra stain. However, it sounds as though she has been rather aggressive adn I can see that this would have put you off side. She really should have made all her expectations VERY clear in her house rules and/or manual - especially as she would have been able to see you were a first timer. In her defence, unfortunately there are many guests quite unlike you who are pretty awful - that probalby explains why the host reacted the way she did, but still does not excuse her manner. There are also many hosts who would have overlooked it, PARTICULARLY if, as Sarah suggests, you'd let the host know straight away. You tend to get crosser as a host if you think guests are slinking away without admitting to stuff.
I'm not sure if you have left each other reviews yet. If not, you'll want to try and make sure you dont' get a terrible review from this host - that will affect your ability to book in the future. if she hasn't left a review yet, If I were you I'd message her adn just explain your confusion/ lack of understanding as you did so well here on the forum. That combined with the fact that you have willingly paid some money should have a bit impact I would think.
I do hope this hasn't put you off airbnb. There are some wonderful hosts and some fabulous places to stay.
Good on you for being so open minded - hope that doesn't sound patronising - not intended.