If I wanted to sell something, excess, I'd ask for the sunk buy price + shipping cost. Most of us on this thread have been good at picking things up on sales, so odds are that our sunk cost is going to be as good as someone can expect to get something for.
I PIF a fair amount, and reuse bubble mailers that I get from Amazon and other sellers. Up to 4oz weight only costs about $2.75 (using Paypal multi order shipping - anyone with a normal PayPal account can use it, you just have to know the link to it), though by the time you're shipping 10 packs of heads the small flat rate box might make sense.
I don't bother with insurance; PayPal shipping always includes tracking, USPS is reliable, and for things like coils the risk of damage in transit is negligible.
I've done the above for items up to full kits. I do keep some spare bubble wrap around to smooth out packages, again using wrap from packages already received.