You may want to closely inspect your labels after printing, I have found in my case at times the quality of the print is less than optimal and yes that can cause errors in the system... I am by no means saying this is the case, however I do like to make sure I've eliminated all possible error from my end.
Things such as printer wear and tear, running low on toner (for those with laserjets), quality of labels (especially if you started on a new batch of labels recently). Are the labels sticking to the packages good, I also like to rub my thumb over the printed area as a "smudge" test to see if the print wears off (it shouldn't).
As well of course I like to see if there's any relationship with the lost parcels being a certain type of packaging, sometimes small envelopes or small boxes like to cause problems too, falling between the cracks as they say... There may truly be no rhyme or reason but if a certain package is what is getting lost then perhaps using a different box / envelope to mail those specific orders can alleviate the issue.
Not sure what else, I know it's frustrating, not even just the money but the resulting defects; it is good you are paying attention to this, if all else fails raising your prices a bit should slow your sales down while you see about finding a solution.
Also I don't refund without an INR, no way. Doing that may give ebay the impression that I'm taking sales off site so I always have my buyers open an INR (instead of refunding I ask them to do that) and then I wait to the last day as a fair half the time the package shows up and I get to keep the money but I can most definitely appreciate you not giving your buyers a hard time.
