Let's say you have the following customers with these amounts owed:

 

 

When you print 30 day letters:

Brown will get a letter with a balance due of 150.00 (100.00 + 50.00).

Black will NOT get a 30 day letter since they have a 60 day balance. 

Green will NOT get a 30 day letter since they have a 90 day balance.  

White will NOT get a 30 day letter since they have 60 and 90 day balances.

Yellow will NOT get a 30 day letter since they also have a 60 day balance.

Pink will NOT get a 30 day letter since they also have 60 and 90 day balances.

 

When you print 60 day letters: 

Brown will NOT get a letter.

Black will get a letter with a balance due of 75.00.

Green will NOT get a 60 day letter since they have a 90 day balance.

White will NOT get a 60 day letter since they also have a 90 day balance. 

Yellow will get a letter with a balance due of 130.00 (60.00 + 70.00) 

Pink will NOT get a 60 day letter since they also have a 90 day balance.

 

When you print 90 day letters: 

Brown will NOT get a letter.

Black will NOT get a letter.

Green will get a letter with a balance due of 150.00

White will get a letter with a balance due of 90.00 (20.00 + 30.00 + 40.00)

Yellow will NOT get a letter.

Pink will get a letter with a balance due of 170.00 (80.00 + 90.00)

 

This logic insures that no customers ever get more than one letter. Instead, they get only the letter representing the oldest overdue category.