How do I lodge complaint by mail to Apple?

My MacPro fails to sync iCloud related services. I tried support, after various troubleshooting calls they sent a log file to Engineering. This was done 6 weeks ago.

Since then no response from Apple, and my attempts to contact them for feedback remain unanswered.

Today I lodged a complaint with the senior advisor, and was told: I will not get feedback and it will make no difference to the outcome of the troubleshooting. Also, senior advisor could not/would not? send the details of the complaint to me for records, and said that if I record it that she will end call.

I wish to send formal complaint in the hopes that it will escalate to problem to management structure so they can actually take action - or refund my purchase.