GDPR isn't a relevant precedent as there isn't a precedent -- GDPR hasn't been challenged in court, not the least, a US court.

GDPR also has no relevance for US interstate commerce.

South Dakota vs. Wayfair overturned the Quill decision, and, the Quill decision would have been the controlling precedent in any constitutional challenge, based on physical nexus. However, that all changed with Wayfair.

One of the key arguments in the overturn of Quill were that Quill created, rather than resolved market distortions. Essentially, Quill (according to the ruling in SD v Wayfair,) created a tax shelter for businesses that limit their physical presence in a state. The problem with Quill is that it disadvantaged economically identical actors for arbitrary reasons -- a small shop with a few items in a warehouse in a state would have to pay taxes on everything they sold in the state, while a large business (such as Wayfair) with no physical presence in the state would be advantaged by not having to charge a tax on the exact same items. Helping customers evade a lawful tax unfairly shifts an increased share of the taxes to those consumers who buy from competitors with a physical presence in the State.

"Quill’s physical presence rule has limited States’ ability to seek long-term prosperity and has prevented market participants from competing on an even playing field."

So now, we look at the California law requiring online cancellations. It would be an extremely difficult argument to cite SD vs. Wayfair since the online cancellation requirement isn't creating a disadvantage on market participants or the states themselves. Lost economic development due to the imbalance created by Quill was cited repeatedly as a flaw in Quill. An online cancellation requirement is going to be very difficult to prove as having a deleterious effect on economic activity/growth/development as was clearly demonstrated in the majority opinion for Wayfair. Wayfair was essentially about tax shelters. The California law doesn't correct any particular economic harm.

However, that being said, any constitutional challenge to the California Law would likely fall short because the Supreme Court interpretation of the Commerce Clause essentially says that laws ought not present an "undue burden" on interstate trade. Allowing for online cancellation, would be, to say the least, a stretch to argue that it places an undue burden on doing business with people within the state. That's why, if this law were to go to court, it would be upheld since someone like the New York Times would have a hard time proving an "undue burden." Ironically (to me,) being required to collect sales tax in all 50 states as well as the thousands of individual jurisdictions (including multiple jurisdictions even within cities, thanks to economic reinvestment zones) -- that to me, seems like a massively undue burden. However, as the court rules in Wayfair, that burden is less important than the economic discrimination that was happening (i.e. a local online shop vs. an out of state online shop being burdened unequally despite selling the exact same product.)

As an aside, I might argue that a business without a physical presence in the state isn't availing themselves to the infrastructure or public services of that state, however they are being asked to pay for those infrastructure and services. Shipping companies actually do the deliveries and they, of course, are paying the taxes for their locality. But that's another debate. Incidentally, justices Kagan, Sotomayor, Breyer joined Roberts's Wayfair dissent, in which he stated that Congress ought to be making the laws and not the courts. I never thought I'd agree with Kagan and Sotomayor on anything, but it is strange times in which we live.

In anyone's interested, Ginsburg, Alito, Gorsuch, Thomas, Kennedy were in the majority on the Wayfair case while Breyer, Sotomayor, Roberts and Kagan were in the dissent. http://www.scotusblog.com/case-files/cases/south-dakota-v-wa...

It was a 5-4 but not how you would have expected!

Anyway, to the point, Quill would have made a challenge to the California law rather easy, but now, the California law will likely pass a Constitutional challenge based on the "undue burden" test established by the courts.