I received the parcel this morning. I was a bit surprised when I opened it that you did not enclose a book with the goods because I had mentioned in my answer to your last email that I preferred a book over a discount code . After reading the message in the parcel I realized that my answer had not been sent and was still in my draft folder .
"had mentioned or mentioned" in this context.
Does "I mentioned" conveys the meaning that my reply was done after the seller's last email and before the seller sent the parcel? .
I think "mentioned" is enough to convey this meaning since "because" is mentioned .If an event is the cause of something it is obligatory that the cause (my answer)was written before its result ( the surprise of finding no book enclosed).
So the sequence of events seems to be clear without past perfect