How binding is a spay contract if written on notebook paper?

A contract written on a piece of notebook paper can certainly be valid. To determine whether or not your particular contract is valid, though, and what the consequences of breaching that contract would be, I would suggest you take it in for a consultation with a lawyer to review it with you. It seems like the first order of business would be to take the puppy to the vet to find out if she's pregnant. That will be a very important thing for everyone to know before any discussions about the contract are had and decisions are made.

It simply does not matter if a contract is written on a piece of notebook paper, a napkin, or even just in an email. Contracts in many cases can also be oral. What matters are the terms, which in this case may or may not be enforceable.