I currently work in the procurement department of a major defense contractor. This company likes to throw its weight around, but we never (to my knowledge) intend to not pay our vendors - our Terms and Conditions are just exceptionally onerous; but I digress. I would recommend, in any communication with a large customer (such as my company), to be as formulaic and procedural as possible. An example of a letter that would draw our attention and get you the result you want would be:
"Dear [Customer POC],
Your account has a past due balance of $XXXXX.XX. This payment was [agreed/scheduled] to be made by XX/XX/XXXX, in accordance with [our/your] Terms and Conditions. If this balance is not paid before YY/YY/YYYY, we will follow with the appropriate legal action. Please send payment to [address].
Sincerely,
[you]"
You want this to look official and direct. Be brief, and don't be afraid to reference legal action even if you don't intend to sue; "appropriate legal action" leaves the next step up to your discretion while still making a material threat to the delinquent customer.
If your customer is a single person, or a very small business, your more emotionally targeted approach will work well. However, with large companies, you need to give them clear-cut rules with consequences outlined if they should break them. Lawsuits are expensive for big companies as well.
"Dear [Customer POC],
Your account has a past due balance of $XXXXX.XX. This payment was [agreed/scheduled] to be made by XX/XX/XXXX, in accordance with [our/your] Terms and Conditions. If this balance is not paid before YY/YY/YYYY, we will follow with the appropriate legal action. Please send payment to [address].
Sincerely, [you]"
You want this to look official and direct. Be brief, and don't be afraid to reference legal action even if you don't intend to sue; "appropriate legal action" leaves the next step up to your discretion while still making a material threat to the delinquent customer.
If your customer is a single person, or a very small business, your more emotionally targeted approach will work well. However, with large companies, you need to give them clear-cut rules with consequences outlined if they should break them. Lawsuits are expensive for big companies as well.