Reply by Loren

Actual weight is a better deal generally. With experience you’ll learn
to estimate actual (packed, padded and boxed) shipping weight
within a few pounds based on real weight of the item.

If you have a lot of the same items figure the cost of new boxes
or new cardboard flats into your costs. Salvaging boxes is great
but it can be a bottleneck. You can build better cartons with
moisture-activated tape and fiberglass-reinforced tape than you
can with standard packing tape.

New box costs can mess you up so on the larger items your
profit margin may be in your ability to build cartons.

... kinda like cabinetmaking, but with sloppy tolerances
and floppy boards.