Notes to the letter:
Do not contradict to the customer and do not tell the customer is wrong. Try to persuade the customer that the damages that have occurred to the canvas sent by our company are exceptional and do not match the practice of our company.
Stress the confidence of the company in our customers. Hence, avoid implicit accusation of our customers of lying. Consider re-wording of your sentence “I find it difficult to believe because we are so careful about shipping, but if what you say is true…”
Avoid your personal views and opinion, while writing the letter to the customer (If you find it difficult to believe that there are sags on the canvas, it is your personal opinion and you cannot state it as the matter of fact) Consider more polite form and stay positive (for instance, consider expressing our sincere regrets about sags on the canvas of our customer
You shouldn’t blame the shipper since the customer should stay loyal and confident to our company. In addition, it is our company’s responsibility to manage shipping of products to customers.
The detailed description of our regular measures used to protect paintings from damages is a very good idea. You could elaborate more on it to enhance the sense of reliability of our company and shipping and to regain confidence of the customer.
Consider effects of your assumptions that refunding costs for the re-stretching of the painting of the customer is against policies of our company. Wouldn’t such a statement undermine the confidence of the customer in our company. I would highly recommend you considering re-wording to enhance the confidence of our customer in our company. For instance, you may present the refunding as a regular policy of our company in relation to loyal customers.
“We are proud that we can offer fine works of original art at incredibly low prices” is a very good point.
However, again, when you assure the customer that he/she can be certain that our company do not send out sagging canvases you may make the customer feel uncomfortable since the customer has already received the sagged canvas. Why don’t you reword this sentence? Consider the possibility of assuring the customer that he/she will never receive the sagged canvas from our company and that our company will always refund damages of canvases that have occurred before they have arrived to the customer.