@TT I believe with the new upgrade option the fonts available are the fonts via the type kit font editor via the WP.com design dashboard....
Staff removed the old option for type kit that required a generated user code... Support document about the new → font options
I’m already using Typekit fonts on my blog. How will the Custom Design upgrade affect me?
You can still make changes to your Typekit fonts, but only via the Typekit.com website — like before, as long as you don’t change your “Kit ID” — the number that ties your account to Typekit. If you want to change that later you’d need to upgrade, but you can leave it as is if you’re happy with your fonts and don’t mind managing them on Typekit.com. Purchasing the upgrade would give you access to the visual font preview and chooser tools, so you can apply the fonts right from your WP.com dashboard.
If you choose to stay as you are, you’ll be grandfathered in forever.