Marvelous invention Balikbayan Boxes
Love 'em

All good tips on how to secure, strengthen and waterproof

Just be careful on that weight, many carriers have tightened up on limits. Might be a good idea to double check first.
Haven't had a pick-up with 2 people for years.

Last week we had a pick and the guy told me he'd just refused one. Said it was almost full with tinned foods and he couldn't move it so left it.

I've also sent a few lined with plywood. Great stiffener.

Some outfits will actually custom-build one for you (at a charge).