With Neo4j Aura, you are charged on the 1st day of the month for usage in the prior month. It is always true that when you delete your last database you have usage that will be billed on the 1st of the following month.
For example, if you have a database with 8GB of RAM running at the start of the month, and you delete it at 2pm UDT on the 3rd, you will have accrued 496 GB-hrs of usage in the month:
(8 GB * 48 hrs) + (8 GB * 14 hrs) = 496 GB-hrs
Even though you've deleted your database, that usage still remains to be billed, and it won't be billed until the 1st of the following month.
If, however, you feel you've been billed in error, please do contact our support team by filing a ticket.