In my experience : just use the best service available from any company, and always insured. I get all my cardboard cases/packing made by professionals who send stuff literally around the world (for the cost of 50/70 euros). Shipping a one K bass to the Uk from belgium is about 100 euro in total. Is it expensive? Yes of course, I am not gonna deny that (shipping a wal to finland was 200 euro in total with UPS). If the buyer is a bit reluctant to pay those kinda cost (all my buyers can attest for outstanding packaging), i'd rather get a bit in my pocket to have a safe sale...
nothing worse than having to claim anything with UPS or any shipping company, as they will use any reason to screw you up ( to them a cardboard case can't be used twice : for all people using bike cases, if anything happens they will claim it is "not safe packing we are not responsible for anything, as if you re use a case, the carboard is weakened"
Not meaning to be patronizing at all ... but just play it on the "safe side".... , and don't waste anytime with interparcel or any kind of weak company like that... there's so many horror stories about those