It's a good idea to let the customer know why there's been a problem—
告知客户为什么会有问题是个不错的想法—
it shows you are prepared to be honest, and you might win a bit of sympathy.
这表明你很诚实，你或许能赢得一点同情。
They realise that things don't always go right for the supplier either. But the most satisfactory thing to do is to offer a refund.
他们意识到供应方也不可能事事顺利。但最令人满意的做法是退款。
Then, of course, they are more likely to deal with you again when they need a replacement.
当他们需要替补时，他们当然更有可能再次找你做生意。