The "changes" you made to the contract would likely not be enforceable by you if it came down to it.
Why not? I cross out pre-printed sections all the time. If they later claim the pre-printed section is the contract, I would argue the document they have signed is the contract. Who would win? If there is a later claim there was, therefore, not a meeting of the minds, then there is not a contract.
At least that's my understanding.
I don't think that, unless there is some form of acceptance like the clerk watching me as I cross out the portions I don't like and then rings up the sale on the register, the business must accept the changes I make. That's what seems to be happening here. (Even though the OP says the business "accepted" the contract.) How do we put things back to where they were?
Especially if we're talking about a new heater in the winter in a part of the country where there is snow on the ground, there could be a good argument that the terms were ones of adhesion and I find it unlikey a court would find them to be part of the contract if the OP crossed them out before signing. (Not that we know because the OP did not follow the simple instruction of listing his state.)
I am unsure of the outcome in this case, but I like the OPs intended strategy of telling them to accept the contract as written or to pound sand and cancel the whold voidable mess. What will they do then?