Why...Faisant suite à ta lettre, J'ai pris ma decision...is wrong?
Apart from using a capital J (should be lower case), it should be accepted.
I did the same thing, but I think they were trying to say that that sentence was clearly formal (business style) and called for the formal "vous."
I wrote the same thing and got it wrong - reported 7/17/18
reported again 22 mars 19