We’ve had the exact same problem as @dmorgan20 – after a a lot of testing and back and forth with them finally got to the bottom of it. An engineer told me that they no longer support WordPress with their e-commerce platform for new users (only Shopify and a couple of others). This changed a few months ago apparently. Plugin is still in repository for legacy users. Only way to do it now is just use standard external links to products on iZettle (or change to a different platform that they support). Hope that helps!
