Good day [recipient],
In the light of rising production costs, we are sad to let you know that we will be raising the amount we charge for [product].
This is due to the recent hike in the the price of [material], which we use in producing [product]. Believe us when we say that we tried our best to fight for the lowest price we can possibly give you.
The new price [amount] will take effect starting [date]. We hope that this doesn’t strain our good relationship with you.
[Name of sender]