I think the tricky bit is less proving you bought something, and more proving that the thing you bought cost more than it otherwise would.

As I understand it (which isn't a lot), if you paid a tariff on an overseas order you're theoretically due it back, although that might require taking the government to court, which is gonna cost more than the settlement for most people.

you would need to prove what part of the amounts you paid were due to tariffs, and which were ordinary price changes. All vendors would need to publish that information, and be honest about it. Don't see it coming.