I need a format or sample of letter of preparedness to pay for service charge.

Financial preparedness? Why the letter? Just pay the service charge.

In a situation where the letter is required before the payment, how will one go about it?

In a situation where the letter is required before the payment, how will one go about it?
Is this homework, for a class?

It is called a letter of intent to pay.

Letter of Payment - Sample Letter of Payment (http://howtowritealetter.net/Letter-of-Payment.html)

Or

https://www.rocketlawyer.co.uk/documents-and-forms/Letter-proposing-payments-in-instalments.rl#

And here:

How to Write a Letter of Intent (with Sample Letters) (http://m.wikihow.com/Write-a-Letter-of-Intent)