We have 15 brands that we have now all migrated over to Shipping Easy -- unfortunately, we weren't aware that Shipping Easy has NOT updated their API to be compatible with Shopify's updates, meaning it causes post purchase upsells to be "partially fulfilled" or "on hold" indefinitely.
Long story short, you can't use post purchase upsell apps (like Zipify OCU) with Shipping Easy. We have had to manually go through every single upsell order and issue new orders (and eat the shipping cost) for every order that was upsold because Shipping Easy has not updated their API.
Please make this a priority feature moving forward. Not having the ability to do post purchase upsells is a massive revenue loss for all of our brands. We won't be able to continue using Shipping Easy for all of our brands long term if this isn't resolved relatively quickly.