Dear {Recipient},
I am terribly sorry that I damaged your {item} on {date}. While the incident was completely accidental, I take full responsibility for my negligence and I'm willing to cover the cost of repairs. You may send the bill or receipt to {email}.
I apologize again for my carelessness and the trouble this has caused you. I can imagine how frustrating it must have been to have such a nice {item} ruined. I promise to be more attentive in the future.
Best,
{Your Name}