We do not charge for heat based on the external symptoms. Instead, we charge this based on the internal condition of your pet's uterus and other reproductive organs. One sign of being in heat, internally, is a very inflamed uterus. This can last up to 3 months after your dog's bleeding portion of the heat cycle.
For cats it is harder to tell they are in heat based on external factors, so the internal condition is what helps us determine this the most. Cats are also opportunistic breeders and cats can go into heat due to a male cat releasing pheromones nearby. When pets are in heat it causes a higher risk and additional time during surgery, which is why there is an additional heat charge.