What is fifth third prox payment terms?

To put this simple.. usually (you can double check with the company) when an invoice is put in these terms it means that the amount owed is due by the 3rd of the following month, but the company is allowing until the 5th to receive payment. This is two extra days before they begin to charge late fees.

5th 3rd prox means you pay the invoice on the 5th day of the 3rd month following receipt of the invoice. The average is 83 days.