See here for details of all 3 plans http://store.wordpress.com/bundles/ offered by WordPress.com.
See here for details in regard to all available upgrades. http://en.support.wordpress.com/upgrades/
See here for ecommerce details http://en.support.wordpress.com/ecommerce/
You must be logged in as Admin http://en.support.wordpress.com/user-roles/#administrator under the exact same username account that registered the blog to access the blog's dashboard and purchase upgrades at > Store > My Upgrades
Your billing history will be at Dashboard > Store > Billing History
Your options for payment are found here http://en.support.wordpress.com/payment/
Note that the free plan and the Premium plan do not allow you to upload your own themes or plugins. WordPress.com blogs cannot be equipped for eCommerce transactions unless they have the $299. per year Business upgrade. http://en.forums.wordpress.com/topic/ecommerce-is-now-available-on-wordpresscom?replies=1
A self-hosted WordPress site might be right for you. WordPress.org offers free software that you can install on a web server. You can upload and install themes and plugins, run ads, conduct ecommerce and edit the database. Learn about the differences here. http://support.wordpress.com/com-vs-org/