I always make my own mailers, or reuse old ones. I always add more crossplys of corrugated cardboard and enusre they are bigger than 12" x 12" to lessen to chance of corner crunches.
I get many positive comments about the sturdy nature of my packing. It takes a bit of effort though.
I never use mailers that are only just bigger than the record sleeve, asking for the corners to get bashed.
As others have said, take record out of original sleeve.
The most expensive L.P. I have posted was about £160 - and I still used recycled stuff to package it.
Lots of 'Fragile' tape.
Write 'fragile' and 'do not bend or fold' on the package.
Shake your package. Are you getting any movement within the package, however small? If so, take it all apart and start again. Nothing worse than contents moving about, grinds my gears...
Make sure the postage covers the cost of replacement in terms of insurance. I have made a claim through Royal Mail and they paid up.
Take photographs of the contents and the package.
Anything over £15 I usually insist on tracked (and signature on receipt) postage. Set your own limits. It can depend on how rare a record is, rather than its value.
I have just sold a 7" to a guy in Texas. The record cost him £10 and the postage has cost him just a bit more ( £10.07 ) - Royal Mail international tracked and signed, classed as a "Large Letter" - that may sound overkill re postage, but there was only my copy for sale, a rare thing.
Keep your receipts for posting in order and add a note to remind you of what the sale was.