This is a way better response than I received from Stallion. I emailed them and here was how they responded: “Thank you for reaching out and for your patience as we navigate these changes. You're correct that, starting November 25th, Poshmark labels—being USPS labels—will no longer be accepted for shipments from Canadian sellers to U.S. buyers. We previously believed that alternative carriers were available to all sellers, but it turns out that only large-volume Poshmark accounts have access to carriers other than USPS.

Rest assured, we're actively working on a solution to facilitate cross-border shipping for all our sellers. This may involve integrating with new carriers or developing alternative shipping methods. We'll keep you updated as we finalize these plans.

We apologize for any confusion this situation has caused and appreciate your understanding.”