https://www.epo.org/en/service-support/faq/myepo-services/central-fee-payment/automatic-debiting/what-if-there-not-enough

What if there is not enough money in my account to cover all fees due on a given day?

If on the decisive payment date there are insufficient funds in the deposit account to cover the fee(s) due on that day, the automatic debit is not carried out and the account holder is informed accordingly by email. 

Where more than one application is affected by the lack of funds, fees are debited in ascending order of application number and fee code. Please note that as a general rule, the EPO processes debit orders immediately upon their receipt, provided there are sufficient funds in the deposit account. Automatic debit orders are processed at the end of the day marking the decisive payment date in accordance with point 4.4 AAD. 

If the deposit account is sufficiently replenished to enable each fee to be paid, the EPO debits each fee automatically. The payment is considered to have been made on the date on which the deposit account is duly replenished (point 7.2 AAD).