Gridview would do it.

Create a View based on the AR Transactions table. Select all the relevant fields that you need (ie. cust number, amount, description, etc).

Create a filter along the lines of:

AND(
{Trx Type} = "IN",
{Terms Description} ="NET 21",
QUERYDATE("Enter Transaction Date")<={Trx Date}
)

The Querydate command will allow you to input a date, say 1 month prior to today, to limit your results to the most current transactions. There's likely a way to automate this further by using a "Today's date - 30 days" tpye of formula, but I can't recall it at the moment.

You can then sort the grid by transaction date. Run this GV daily, or weekly and print it off to provide a reminder for your AR people to process the payment.