Do I need a copywriter for my online store?

the product descriptions, you'd do well to optimize each page for whatever product/keyword you'r trying to sell

I would argue that in some cases, having good copy is more important than having a good product. Take a look at the Acai Berry products that are posted throughout social media. It doesn't matter that the product probably won't help you lose weight. The copy makes it seem that it WILL increase weight loss by citing relevant statistical data AND appearing as if it was written by your friend. Real copy is a an art form. Unfortunately, the internet makes it seem like anyone can do it.

My opinion is that you have to optimize every little text and content you have on your page. It is easier to double the sales from 2 to 4 sales per 100 visitors than stay on 2 sales per 100 and to double visits from 100 to 200 visitors. So the better your copy -> the more efficient you are!

You ALWAYS need a copywriter. In all honesty, if you're going to skimp on anything, it should not be content. You're better off with a very basic website with great copy, than a custom site with terrible content. It's crazy how so many IMers underestimate the value of good copywriting.