Formal letter (help)

Hi everyon,
my name is hikarin Aisle and am new to this forum.
First i have to say this is a great forum and helped me a lot in the past few days.
But i have now a problem, i am trying to learn to write formal letters and i have got some exercises but there are no solutions.

If you are so kind and help me with it? (and sorry for my poor english)

Here is a informal memo and i need to write a formal one and i am not sure if everything is correct. I still have difficulties with those formal phrases or does anyone have any tipps how to study it?

Some of you have been cutting the price for the guys who buy our stuff in Britain and sell it to the shops there by more than 25 %. Sometimes even 30%! That´s to stop!

First, it has come to my attention that our sales representatives have granted our wholesalers in UK a trade discount of 25 to 30 percent. (thats to stop don´t know how to write it)

Don´t give those guys any extra time to pay either! They have got to pay right at the start, as soon as they tell us they want the goods.

Second you are not allowed to grant extensions to credit periods. We will change our current terms of payment to cash with order.

Informal Phrases:
And another thing: never tell them you will get the goods from vienna to london, and taker care of transport, insurance, etc. We only get the goods as far as Hamburg - and onto the boat there, of course. But no More!!!

Formal :
Finally, we will neither deliver our goods under (or on which is better?) CIF nor DDP terms anymore.
We will therefore only offer EX-Works or FOB terms.

Informal :
And on the other issue: you need to send us the bills for things you pay for while you are aqay QUICKER! Sixth of the following mont, no later " We don´t get the bills, you don´t get your money back.

As regards the second point I mentioned at the start of this memo, you have to submit your travel expenses (or receipt?) at the 6th of each month in order to be reimbursed.

thx in advance