You wish to close your personal Heroku account (not a Heroku Team account).
If you have any ongoing charges, be sure downgrade your apps to Free Dynos and delete all add-ons (except the free Heroku Postgres). Alternately, you can just delete your apps. You can do any of this from your Heroku Dashboard or the CLI (as you prefer).
Once that's done, you can use the Remove Credit Card feature on the billing page to expedite your final payment. If you have any remaining ongoing charges, this feature will let you know what apps they're stemming from.
When your final invoice is paid, you can proceed with self-closing the account.
Please note, closing a personal account will not close a team account. Closing a team account is a separate process, and should be done first if the objective is to close both your team account(s) and personal account.