It’s common to receive a mail from some service that they’re changing the terms and conditions of their service (internet provider, gmail, etc.) These changes are unilateral and can be about anything, from their privacy policy to the money they intend to charge me.
Why can’t I unilaterally send them a mail and say, I am changing the terms of my patronage and will now insert my new policy (pay only xx amount, or demand a mint Chico chip ice cream every Tuesday)?


you can! if they agree to that contract. which is incredibly unlikely.
NOTE - This happened in russia
https://detechter.com/man-creates-credit-card-sues-bank-for-not-paying/
I tried doing this with my landlord. I moved my business into a building with the understanding that after a year I would buy it. Around the year mark the LL started dragging his feet about the Salé and started to say he was going to open a business in one of the vacant suites, or boot a tenant he kept having issues with, and open in their suite.
Then the disliked tenant had a fire that severely damaged the building. While repairs were in progress the LL agreed to sell the building to the problem tenant without any word to me. The sale was over a year out when I had to renew my lease. He sent me a word doc to sign, it had his signature already added. I added a clause saying if he was to sell I had to decline in writing before he could sell. He only noticed because it changed the page count of the lease.
I really wish I had added it to my initial lease.
Imagine scaling down the font and them not noticing… would’ve been glorious.
Unfortunately (or fortunately, probably) contract law doesn’t really work that way. If you sneakily change a contract or even verbally misrepresent what’s in that contract, it can be challenged in court.
It can be difficult to prove though. For example, if a car sales person verbally says your car payment will be $100 but the contract you sign says $1,000 then you can argue in court that they misrepresented what you signed and the $100 should stand. But it’s hard to prove what was said was different than what was signed.
Same concept if your landlord signed something, sent it to you to sign and you modified it to something else without communicating a change. If they can show the original document they sent you, you’re boned.