There is nothing to be confused about. You buy a domain name and that's all you get - a domain name. You buy domain mapping and that's all you get - domain mapping that ensures your visitors are seamlessly transferred from the wordpress.COM root blog to the URLs to the same posts on the domain.
Every upgrade is only for what it clearly states. there are no additional perks. Each one is specific and applies to only one blog, and must be renewed annually.
There is no upgrade you can buy so you can have advertising on your blog. The only exception is for very high traffic blogs that apply and qualify to pay for VIP hosting.