One of the best things you can do, once on page seo is sorted, is to create a blog on a subdomain, and use it to go into specific details of pages you would like to rank for.
For example, doing a blog post on a specific product/category that links to the product/category page. I rank no1 for chosen keyword with only 2 links from 2 different specific blog posts only using naked url links (no anchor text) + Some more specific posts that link deep into those chosen categories.
Also as these pages/links are highly relevant, they will stand the test of time, perfect if your e-commerce store is looking to develop into a brand name, as your links are going to last, and you should be consitently ranking for chosen keywords.
If you want anymore help pm me, I am 100% self taught, and have never purchased SEO services, you don't need to either.