A little thread drift.
WHY do they allow bottles of duty-free on planes?
Wouldn't it be much simpler, safer, more sensible to have a duty-free chit/voucher redeemable at the destination?
Less aero-drunks, safer in a crash, and the biggie: How far do you have to fly before the fuel-usage is a greater weight than the bottle?

I'll bet someone out there has the formula for that answer.