Once you have upgraded to a paid plan from a trial account your billing period will begin immediately and your credit card will be charged for the first period. Each subsequent periods after the first you will be charged the rate of the selected plan.
We'll send bills to the email address provided with the billing information. If you can't find it, please look into the spam folder as well and consider marking firstname.lastname@example.org as a safe sender so next emails won't be classified as spam.
The bills can be downloaded from the workspace's Manage / Manage subscription menu as well.