There was a time when I was sending a certain number of items as large untracked letters and doing that same as you by only sending tracked if I though there might be an issue.

However, I decided to change things as I was getting a few "not received" queries last year.

Now I use the large A4 sized stiff green envelope which is tracked, for around $5.20. People are still buying at the same rate and I have peace of mind and so do my buyers. 

I have a number of postage policies which cover the different types of shipping I offer and I just changed the policy for those items from untracked letter to tracked letter, which also now complies with Ebay's valid shipping requirements.