Add accounts any time. You're charged the prorated difference for the rest of your current billing cycle, and the new account limit applies immediately.
Remove accounts any time. The change takes effect at your next renewal — you keep your current limits until then, and the remainder of the current cycle isn't refunded.
Cancel from your Porter account any time. If it's within 14 days of purchase, you're eligible for a refund. After that, canceling stops future renewals, but the current billing cycle is not refunded.
Disconnect their account and the slot is free immediately for another client. To actually pay for one less account, remove it from your license — that takes effect at your next billing cycle.
Nothing is billed automatically beyond your plan. Extra accounts or credits are always something you choose and pay for upfront.