It probably wouldn't have to be anywhere near 25% at the national level. Total US retail sales are what, about 7.3 trillion? A small percentage is a big number.

Oh, it woujld start small. And then grow like Social Security did. And like the Federal income tax did. Both of these started small, and then incrementally grew. How to boil a frog.
Google: UK VAT started at 10% and is now 20%.

I saw how it works twice in my lifetime.
"We need a x% tax surcharge to pay for this storm damage. It's not a tax increase because it's just temporary."
(Later): "The new tax rate will be X% more. This is not a tax increase because it's the same as you've been paying for the last few years."

I'm pretty sure it wouldn't require a constitutional amendment in the US

No, it wouldn't. That's why I said "Ha!"

It would be a previously invisible & unknown part of the Constitution, found in the shadows of a penumbra. Like Wickard v. Filburn "Congress can regulate any activity that has a substantial economic effect on interstate commerce."
Or the previously unknown rights of abortion and gay marriage. It was there all along, it's just that nobody noticed it.

A D- average first year law student could do it in his sleep.

Anybody who reads history knows how it all eventually ends.

But none of this matters here, because nothing we say here has any effect on what the governments will do.
I guess we get into these discussions because there's not a whole lot new to say about Berkshire Hathaway. 'sokay, boring growth is good. ;-)