You should be careful just adding "not", because they can always claim they didn't notice you modified the agreement (I'm assuming no one on their staff signed or initialled your change).
Isn't the worst case that the contract is just invalid? If you made a change which they didn't see, then there's no meeting of the minds, and thus no contract. As long as you didn't need the contract anyway (it sounds like it's purely for their benefit), it's all the same.
The European custom is to ask the other party to sign on the border of the lines where changes have been made, so they cannot claim that they did not notice a change made to a canned form.
Yet, this allow the signing person to add _other_ changes once the signatures are made, but this custom is useful in the classic 80% of the times (maybe 99% in this case).
I'm not an expert on contract law, but I'm fairly sure that one party modifying a contract without the other person's notice or consent does not make the modifications legally binding, which is why you do need to get the other party to initial changes you've made.
A contract is a physical expression legal agreement which both parties are supposed to know about, it's not a magical piece of paper which enforces whatever's written on it.
I should note that in this particular case the chances of any of this ever mattering are essentially zero. Just letting everyone know, in case they think they can go out, buy a house and move a decimal point in the sales contract while nobody's looking.
So if I don't know what's on the paper because no one has made sure that I do then I still can be upheld on what I signed but if the guy who gave me paper to sign doesn't know what's on the paper because he didn't notice my changes then he can't be liable?
You have a duty to read before you sign a contract. In this case, they can claim you tricked them into thinking you'd signed the form contract, and that they relied on that agreement when they provided you care. IANAL, but I suspect you will lose this unless you can show that the staffer accepted your changes knowingly.
I'm still not an expert on contract law, but I would suggest that yes, that's the case. The law isn't necessarily perfectly symmetric between the original text of the contract and handwritten alterations put in at the time of signing.
