I am so confused lol
When you are setting your rates for additional dog how exactly does it work?
As in if I put additional dog at $0 does it mean the second dog is free or does it mean it gets 0 discount towards it?
That would mean the second dog would be free. The additional dog rate is your fee for a second (or third) dog. You can leave it the same as your regular rate or reduce it. If your regular rate is $20, you can set the additional dog rate at $20 if you don't want to give any discounts, or $15 or $10 or whatever if you want to provide a discount.
I have a rate for additional dogs that is $5 less than my regular rate. I only board one dog at a time, or one dog family, so the dogs are familiar with each other and are used to being around each other. My experience for the past year has been that caring for 2 is not that much more trouble than one; if that changes I may drop the discount rate.
All the rates you set are the rates that are charged to the client. Nothing in the rate section is entered in terms of a discount. Consider it like a menu of services you offer.