I need to write a letter to other standards development organisation bodies that we supply free products to, about payment for postage and courier cost that we want them to start covering since we can no longer be pay for.

Please help me!! How do you ask your customers to start covering for courier charges in the most calm and professional way. This will be a once off payment ($500 )that will cover courier charges for the whole year.

We send this product three time in a year for free since we are of the same industry and we just can't cover the postage/courier charges any more.