Just out of curiosity, is there a large price difference between shipping Priority versus flat rate? For all the lost shipments and time to make replacement sets, it might be worth it to try Priority instead. I'm sure your customers wouldn't mind paying an extra dollar or two on shipping costs.
I've always shipped my items Priority with Delivery Confirmation and haven't had a lost package yet. And I do a fair bit of computer component trading/selling/buying.
In addition, Delivery Confirmation is free if you print it yourself from the USPS website. You can also order free shipping boxes/envelopes through the USPS and they will have them delivered to you.
https://sss-web.usps.com/cns/landing.do