The sentences are correct except for the ones I change here:
2. We took a quote request from a customer who hasn't asked us for a quote for a long time. (If you refer to the Price as something quoted, use quote. Quotation is not wrong. It is just not as common as quote.)
"5.So that we can reduce the unit price. (We can make the unit price cheaper.)"
[This is not a sentence. It is a part of a sentence which should be added to the previous sentence.]
"6. Plus a dealer gave us an email and said that some of our knives which they have in stock got rusted." [This also is not a complete sentence. It is an indirect reference back to a previous sentences. "Plus" is not wrong. It is uncommon. It is an awkward word to use.
You can use "Also, ..." You can use "In addition...." or "Moreover..."
Otherwise, you can skip the transitional term and just begin with; "A dealer gave us an e-mail....etc."
Your other sentences are fine.