Processing fees are charged whenever a deposit (full or partial) of cards is ready to be made into your account inventory. If a processing charge results in a negative account balance, the deposit is placed on hold, the due dates of all pending submissions are postponed by one day, and an email is sent notifying the customer of insufficient funds. Subsequently funding the account to $0.00 or greater will allow the pending deposits to be completed and any submissions on hold to resume processing within a few hours.
Since processing fees are required to be paid before the items are deposited, it's not uncommon for a first-time consignor to incur a negative balance initially as they "get the ball rolling". As long as your items sell for more than the per-item processing fee, your consignments will turn a profit. You can request that a positive account balance sent to you in the form of a cash-out, or you can use those profits to pay for the processing of future submissions in order that you not have to fund your account directly.