I get charged for 2 dogs instead of an additional dog, why is it doing this?

Hello,

I have two dogs and whenever I try to book a dog sitter instead of just adding the additional dog it automatically books it for both dogs. For example, if it's $35/night with an additional $5/dog. Instead of being $40 for both pets it comes out to $70. Is there a way I can avoid this? Thank you!