Allbirds co-CEO Joey Zwillinger has waded further into the ongoing debate over Amazon’s propensity for cloning popular products, penning a short Medium post slamming the company’s copycat behavior and offering to help the e-commerce giant make its version more environmentally friendly.

Amazon made headlines earlier this year when its private label brand, 206 Collective, began selling a shoe that copies the most distinctive features of Allbirds’ namesake runners. Amazon has employed a similar tactic across numerous industries by spinning up cheaper versions of its products across dozens of private label brands that do not bear the Amazon name. That’s gotten the company in trouble with regulators overseas and made it a competitor of some of its most valuable Marketplace sellers, a situation that has also spawned antitrust concerns in the US.

Allbirds is offering Amazon some sustainability tips

In Zwillinger’s post, he calls out Amazon not just for copying, but for doing so cheaply and in a way that does not align with Allbirds’ approach to sustainability. “We are flattered at the similarities that your private label shoe shares with ours, but hoped the commonalities would include these environmentally-friendly materials as well,” he writes. Zwillinger then quickly switches gears to talk about his company’s approach to ethylene-vinyl acetate (EVA), the chemically created foam that lines the bottom of Allbirds’ and countless other shoe brands’ products. Allbirds calls its version SweetFoam.

Zwillinger says Allbirds, working with petrochemical company Braskem, “created the world’s first green EVA — a sustainable version of the foam used on the bottoms of sneakers (including yours), and one of the industry’s most ubiquitous materials.” The company uses sugarcane waste stream instead of petroleum. “We’re also removing carbon from the atmosphere and locking it away with one of the most photosynthetically-efficient crops, fighting climate change in the process,” he explains.

Zwillinger says Amazon can borrow its SweetFoam formula free of charge. “If you replaced the oil-based products in your supply chain with this natural substitute (not just for one product, but all of them), we could jointly make a major dent in the fight against climate change. With the help of your immense scale, the cost of this material will come down for all users of this material, allowing for even broader adoption,” he writes. He goes on to ask Amazon to “please steal our approach to sustainability.”