How to apologize and explain to others in English

Dear Mr./Ms,

We are sorry that we can't send the catalogue and price list you asked for in your letter of March 10 immediately. It is expected that the printer will supply the goods in two weeks, and we will send you a copy as soon as we receive the goods.

Yours faithfully

Dear Mr./Ms,

We regret that we can't send you the catalogue and price list requested in your letter of March 10. The printer will provide it within two weeks, and we will send you a copy once we receive it.

yours respectfully/sincerely

Second, explain

Dear Mr./Ms,

I received your letter yesterday complaining that the central heating system in your new home was not installed on time, which I was very concerned about.

Looking up our previous correspondence, I found that I had made a mistake about the date of completion. This is entirely my fault, and I am deeply sorry about it.

I know our negligence must have caused you inconvenience, and we will do everything possible to avoid further delay. I have instructed to give priority to this work and arrange for the engineers engaged in this work to work overtime. These arrangements will ensure that the installation will be completed next weekend.

Yours faithfully

Dear Mr./Ms,

I received your letter yesterday complaining that the central heating system in your new home was not installed on time, and I was very concerned about it. Referring to earlier letters, I found that I had made a mistake about the completion date. This mistake is entirely my fault, and I'm very sorry about it.

Recognizing the inconvenience caused by our negligence, we will do our best to avoid any further delay. I have instructed that this work should be given priority and asked the engineers to work overtime. This arrangement will be installed next week.

yours respectfully/sincerely