As a pet lover I would consider charging the pet parent extra for repeated offense done in the house out of courtesy! I understand that sweet little pets have occasional accidents. Likewise, if your child got poop on the carpet or peed all over the bed sheets at a friends house you would help clean it up right? The same should be with animals. If you keep a dog that likes to mark then it is not house broken. After multiple trips to the dog park and many long walks if your dog takes a leak inside it is not house broken. However, I understand that it is not fare to the dog to lose his entitlement bestowed to him by his owner and it is not fare to the sitter who has opened up his house to have it saturated in urine.
If the dog is permitted to secretion in his normal habitation then he needs to be in an environment where that behavior is not only respected but welcomed as well. He needs to feel at home and not unregarded when he urinates as he is allowed to do so in his own residence. However some sitters like me are better fits for dogs that are trained and house broken. I have two suggestions; either position your dog's right in the kindness of a kennel where he or she may mark without any hesitation or choose a sitter where the dog can have access to the yard at all times. In a kennel your sweet little pup might be a good fit in a mark-friendly environment that is fully staffed and equipped to draw together after a pet with this particular skill set and need. Not to mention with in home boarding dogs are treated as part of the family and it is a luxury to vacation in such an environment. Different needs for different pups.
I never charge a clean-up fee. Doesn't make sense, lol. Clean-up fees are for Airbnb!