Collection processes are basically at the discretion of the business. You need to have a policy in place regarding collections, because the problems of unpaid accounts are a natural business risk, and often expensive.
The second reminder letter is usually an interim measure prior to actual recovery of amounts due.
It's also a second chance for the client, who may have difficulty paying, which is another consideration in collection processes. You may wish to add a clause about scheduling payments, or other payment options.
This example relates to an undisputed payment.
Reminder of overdue account
If you have already paid this account, please disregard this notice.
I refer to our previous reminder letter regarding overdue payment on this account:
Goods purchased:1 Mega Breeze Air conditioner $699.90
1 Persian Rug, Axminster $150.00
1 Delivery $ 40.00
Amount due: $889.90
Total amount payable $1023.38
Please arrange for the payment of this amount at your earliest convenience.
Important: If you are having difficulty paying this account, it may be possible to schedule payments, or arrange a line of credit, or some other method of addressing your needs.
Please contact us on the above phone number or email if you require further information.