I think part of the answer is the value of the product.  For items below $15, we use untracked mail and over the years the lost or stolen rate is about 0.3%.  With the cost factor of tracked mail vs competitive costing, there is no cost benefit in using tracked mail, i.e. the losses don't justify moving to a more expensive mailing cost vs the potential loss of sales.

Your item not received issue could also be related to buyer honesty.  My experience is when an item is reported as not received and I offer to send a replacement the majority of buyers accept.  The ones that just want a refund cause me concern, because the potential is that they have received an untracked item and just want to take advantage of the eBay policy and get it for free.  My experience is that this happens in about 5% of cases, which I would probably associate with the general level of dishonesty in the community, but I could be wrong.

I believe that eBay should keep track of buyers who regularly report items as not received on a few occasions I have noted people with a bit of a history of this and have reported them to eBay, but in keeping with their 'the customer is never wrong (or dishonest) policy, eBay does nothing that protects the integrity of the platform in favor of the seller.

Unfortunately you will lose a dispute every time without tracking.

I use the green domestic envelopes for items that are thin. They come in 3 different sizes, so worth checking out the next time you're in the PO.

Things like magazines would fit nicely in the A4 size as it is a rigid envelope . You would have to increase your postage to around $6.50 for that size, but you do get peace of mind with the tracking. Much more cost effective to do this than lose your money altogether.

How quick do you refund the buyer. I will usually look at the date it was sent and if its still within the time frame ask if they can wait till end of the week or the following monday.  I have found I hear nothing back and when i message them it has arrived. I find it is very rare for an untracked letter to go missing these days.

Buy tracked envelopes between 3.15 and 6.40 depending on size.  If they don't want to pay for tracking, I don't want to sell to them.  

If you don't use tracking,  you have to live with INR claims. simple