Plugins on WordPress Hosted Sites?

I’m looking into adding a shopping cart to The Bifrost Detour, checking out a Paypal WordPress Plugin that would allow me to do this. However, I’m being hosted on WordPress, and I have yet been able to find the ability to upload plugins (I have a feeling I have to pay some upgrade fee to WordPress to be allowed to do this, since I have to pay an upgrade to be allowed to modify the style sheet). Does anyone have experience with this and is able to offer some insights for me, please? Thank you for your time.