Can you explain why you think SEO is relevant for customer retention? If you are customer and have an account with a shop, you don't need google or bing to find the site, since it's in your favorites or history or password vault.
Do you use google to go to amazon.com to buy something?
SEO is mostly relevant to get new people to your site. And if you have a bad site or not the product they are looking for or too high prices, they go as fast as they came.
The best thing about the SEO is growth and customer retention. You can hire SEO specialist or can make a set up for long to generate more and more lead.