What you describe sounds straightforward. Contract law, generally speaking, is not complex. They teach it first year, first semester in law school
In order to have a contract you need 3 things
1. Offer on the part of one party do do or refrain from doing something.
2. Acceptance of the other party of the offer
3. Consideration...agreement to exchange something of value in return or the agreement
What you describe it sounds like you had a valid contract that you fulfilled.
It is also sounds like you had some conversations with the lady about this rebate.
Now...as I mention, it is possible to modify a contract after you enter into it. But the process requires the same 3 factors...you need an offer (to modify), acceptance of the offer, and new consideration to make the modification
If, for example, you had said "I have this great deal...I will get you a rebate, and do it for you for $10. We can make this our new contract". Had you made the offer and she accepted, then you failed to deliver? Bingo...she has a case.
But that is not what you describe.
You mention small claims
. The formal rules of evidence and procedure do not apply at small claims. It is an "everyman's court"
I would go to court and explain to the judge what happened. Bring a copy of the contract. What you describe? You do not owe her any money at all.