回复：谢谢回答。不是换gutter,只是排水槽上的网

You can not. As I said, you can not introduce evidecen to contradict a written contract. It does not matter whether the contract can be construed as a total integration or not. It will be a different story if you can prove a fraud or misreprensentation. I thought about another possibility if you can introduce the evidence to interopret the contract because it is unclear on what material should be used. You may succeed on this because the contractor wrote the contract (I assumed) so that any unclear parts will be construed against him. This way, you may be able to convince a court to find the contractor a breach. However, it is likely the breach will be found minor because plastic or aluminum, you can not prove which was really better than the other. If it is a minor breach then you must pay. On the other hand, the contractor can argue that there was a mutual mistake because you thought of aluminum and he thought of plastic. He could relied on this to rescind the contact. However, you still need to pay him since he conferred a benefit to you by installing even the plastic one.