You can sue the party in Small Claims Court and get a judgement.  Enforcing and collecting the judgment may by more hassle than it is worth.  If all you want to do is make a point and show that you will not be taken advantage of then go for it.  Send them a letter by certified mail return receipt requested laying out all the facts: when you took care of the pet, how much is owed, etc. and advising that this will be the last reminder before you seek the help of the Courts in collecting your money. Then go and file a claim.