I have purchased many domains. Domain registrars may try to sell you hosting, etc along with a domain name but you don't have to take it.
There are two kinds of emails you can have. One is just where people write to email@example.com and it gets forwarded to any email address you choose. This is the cheapest. The problem is that you cannot use that as an email address to write out to people. So although you can tell people to write to you at firstname.lastname@example.org and their email will be forwarded to your yahoo or hotmail or gmail account, etc, you cannot write back to them with that email name.
For that, you need hosting. If you already have an online store - then you already have hosting. Hosting is your website's physical presence on the web. The domain name is its postal address. Think of your hosting as a warehouse, that stores all your web info. Your domain name is then the postal address that allows the postman to find your particular warehouse among many others.
As you already have hosting, you should be able to set up an email with that.