A formal reply to a customer's rude response

Question

What would you reply back to a customer who has responded back rudely?

Writing 'I'll be glad to help you' in this case sounds vague.

Any alternate sentence you can think of.

Also, we regret for the inconvenience caused is quite common. So apart from that any suggestion?

Answer 1

For a written response, you can start off by saying

I am sorry to hear that....

and then summarise the problem that the customer reported.