Owner didnt inform me her dog was in heat until drop off, how do I tell her that there in an additional fee?

Im very upset and I feel a bit taken advantage of because a dog was just dropped off at 9:15 am for boarding for 4 nights which is fine with me. She is 2 years old and I was told she is well behaved. The problem is that when the owner pulled up the first thing she tells me is that her dog just went into heat in the car. I was really shocked and I was upset immediately though I remained professional and polite, I couldn't quite figure out how I should handle the situation but I know there should be a extra fee. I haven't ever watched a dog in heat but my dog has just finished her first heat last month so I have experience and I've researched a lot of info regarding caring for dogs in heat. Im upset at the fact that the owner probably knew she was due to go in heat soon but made no mention of that when we spoke, I feel like she should've disclosed that info prior to me even accepting the stay. As you all probably know apart for the physical part or their heat there is also behavioral changes, separation, insuring that no accidents happen, etc. that make it 10x harder than a normal stay. I am willing to still keep her because they're going out of town today but I don't think that its fair to me to keep the same rate because of all the extra work and care she needs at this time. How do I tell her that an additional fee will need to be added?