I have a house cleaning service, but need help writing a letter to certain customers asking to increase the cost of their cleaning do to gas, travel expenses, etc?

Send an invoice stating expenses, total the costs and post, then follow up with a call to confirm date of payment.

If you're cleaning homes, then I would like to assume that your at least somewhat familiar and friendly with the owners. If it were me, I wouldn't go through the bother and fret of writing a letter but simply explain in person to them that you are needing to increase costs slightly because of the cost of gas, mileage and the time it take to travel to provide the services that you do for them. I'm sure that they would understand.