You aren't required to pay for a custom domain for your Auctiva store no matter what title you give it, however doing so gives your store a more memorable location your customers can visit. Rather then http://store.auctiva.com/yourusername they would go to http://www.yourdomain.com which also looks more professional to customers and is more attractive to the search engines like Google.
There is a registration fee associated with owning any domain name on the internet, and so it is no different with an Auctiva Store Domain. Auctiva will manage the domain for you and point it to your existing Auctiva Store. Nothing else about your store changes.
Hope that helps!
-- Auctiva David