on tip that may help is a short Thank You note in with your items,

mine is just a copy,

thanking for their order, saying i hope it meets with their expectations, any problems just contact me, I'm sure we can work it out.

I have left feedback for you,  please take a moment to leave it for me, keep checking my Store for more interesting items.

then I add my eBay ID, email address and PO Box number

this works as a packing slip, as well as thank you, and gentle prompt for fb

these days I average about 75% f/b received.