Non-Disparagement Clause handwritten on my receipt?

[Anonymous777]

I recently cancelled a contract with ATA, and was told I had to pay half the remaining amount in the contract. Upon getting my receipt, they stapled another piece of paper to the receipt and read it to me. It said "This sale is final as long as there is nothing negative said about us on social media, or like so. If so, you will be obligated to pay the full amount of the balance." I told her I'm not a gossip, but I'm not signing or agreeing to anything else, I'm just done, and she said she had to give that to me.
My question is, how legally binding is that? I never once signed an agreement that included a non-disparagement clause with ATA, nor would I have. We had a "difference of opinion" and several important issues which led me to cancel the contract. If it's handwritten, attached to a receipt, and not included on any agreement or contract that I've signed, is that legally binding?

 

[army judge]

I recently cancelled a contract with ATA, and upon getting my receipt, they stapled another piece of paper to the receipt and read it to me. It said "This sale is final as long as there is nothing negative said about us on social media, or like so. If so, you will be obligated to pay the full amount of the balance."
My question is, how legal is that? I never once signed an agreement that included a non-disparagement clause with ATA, nor would I have. We had a "difference of opinion" and several important issues which led me to cancel the contract. If it's handwritten, attached to a receipt, and not included on any agreement or contract that I've signed, is that legally binding?

It isn't a question of being legal, its more about contract law.
As you wisely speculated, where is it mentioned in your contract.
Do yourself a favor, take your refund and let it go.
If you must post a negative opinion of this outfit on social media, have someone else do it, or use a pseudonym.