Amazon has a new rule in place governing seed and plant imports for U.S. customers: Nope.

The online retail giant confirmed in a Saturday report from the Wall Street Journal that U.S. customers are no longer allowed to import foreign seeds or plants. Amazon will still sell seeds to people in the U.S., but only if the seller is based there.

The only catch to that concerns non-U.S. residents: If you sell seeds or plants outside the U.S., you can't come into the country just to sell them inside the country. It might be a trickier thing for Amazon to police, but it's the rule all the same.

The policy change, instated on Sept. 3, comes after "thousands" of seed packets were delivered to U.S. mailboxes over the summer, with many postmarked from China. The report notes that it is believed the mystery mailings are part of a "brushing" scam, which aims to artificially inflate a seller's visibility on algorithm-driven ecommerce websites like Amazon.

The site's "plant and seed products" rules page for sellers does indeed note that seeds imported from outside the U.S. are no prohibited, along with those sold by non-U.S. residents.

The source of the mystery seeds that circulated over the summer is currently under investigation. This includes three different federal agencies – the Agriculture Department, Customs and Border Protection, and the Postal Service – as well as various state-level departments of agriculture.

China is also looking for answers since "most" of the packages, which also popped up in Canada and the U.K., bore postmarks from there. China's Foreign Ministry determined over the summer that the mailing labels the country's investigators had reviewed were forgeries.

The USDA has reportedly received close to 20,000 reports of these shipments, and has collected roughly half of them. Agriculture imports are monitored all around the world because new arrivals from abroad could threaten local ecosystems. That's why there's extra emphasis on declaring fruits and vegetables when you're traveling between countries.

Case in point: The USDA's investigation of the mystery packages turned up a number of "noxious" weeds (dodder and water spinach). The investigation also turned up a number of diseases and pests. Those findings "haven't sparked significant concern," according to the USDA, but the investigation continues. The real goal of the mailings appears to be the aforementioned brushing scam.

Seed sales are serious business on Amazon, but they've also been the subject of shady behavior before. Mashable reported in 2019 on the third-party sellers peddling seeds for fantastical, non-existent plants and produce like blue strawberries and rainbow bonsai trees.