Hi, Tony,
"Naming convention" does not sound right to me in this case. I'd use "date format" as well as change the order within the absolute construction (it does not make sense as written: a convention or format cannot BE the date; it is the date that follows or adopts a certain convention or format). I'd also make a few more changes:
Given that you have chosen the second method for filing, I suggest that you create a folder for each supplier and identify each invoice with the date of payment following the YYYYMMD format/using the YYYYMMD format for the date of payment.