When you choose to close / delete your account, we specifically mention that doing so will charge the current account balance for that month to your card, and any unpaid invoices from previous months.
As per the pricing page, for Pushy Pro, the account balance is calculated as follows:
$15 (base cost) + ($0.005 x active device count)
Any usage of our service during the calendar month will result in a charge at the end of the month, or at the point in time which you decide to terminate your subscription, by voluntarily closing your account.