Declared value. When you ship, they ask you to list the items you're shipping, and what they are worth. These go on the customs forms. Boxes can be opened and inspected, so lying is a gamble, but there's obviously a lot of wiggle room here.
The changes are to the commercial de minimis rule, so AFAIK, the personal $800 exemption when you bring something with you still applies, and you might not have anything to worry about at all. Also, when you declare something as "American goods returned", they are not subject to either de minimis rule, even if you send them by mail.
Things you purchased outside the US could qualify as well, if you can prove that you owned them for more than a year while living abroad. But realistically, nobody is going to make a federal case about a box full of old books and underwear...a box full of Louis Vuitton bags and Moncler jackets with tags, on the other hand...