To reference: Shopify Admin -> Settings -> Shipping -> Packages -> Soft Packages
So, the poly mailers themselves are 6 millimeters thick (high), but it's asking me for the LxWxH dimensions. I sell women's clothing, and I'm guessing it's asking me, in general, what the thinkness of my packages will be? It says, "Packages thicker than 0.75 inches don't qualify for USPS First Class Mail rates."
Is the question "how thick is your unpacked poly bag?" Or is it, "how thick will all your packages be, when filled up with products?"
I'm guessing it's the latter? Just makes it difficult because, it depends what I put in it. Some blouses are way less than 0.75, some jackets are thicker.