Your invoice has two possible parts, added together.
The per-seat charge (Standard)
€4.00 per active member, per month — but billed down to the second, not as a flat per-head fee. If someone joins partway through the month, you're only charged for the time they actually had access; the same applies in reverse if someone is removed partway through. You don't need to do anything to get this proration — it happens automatically based on when members were added or removed.
The Plus add-on
If your organization has Plus turned on, an additional flat €40.00 is added — per organization, per month, regardless of how many members you have. Unlike the per-seat charge, this isn't prorated by the second; it's a flat monthly fee for as long as Plus is enabled.
No card required
Organizations are invoiced monthly and pay by bank transfer — there's no credit card on file, and nothing is charged automatically outside of your monthly invoice. See updating your billing details for changing the address or email your invoices go to.