Shipping Easy has not updated to comply with the new Shopify Fulfillment API requirements as of March 1, 2020. This is a deal-killer for us and could easily result in us switching to a new shipping platform.
Essentially, if Shopify puts a hold on an order (e.g. an order is pending post-purchase while a post-purchase upsell app like One-Click Upsell provides an upsell recommendation that could be combined with the original purchase to make an amended, combined purchase), Shipping Easy should pull in the final order with any amendments. Right now, Shipping Easy only pulls in the initial portion of a Shopify order on "hold" and does not add the additional products added with any supplemental purchase. There's not even a way to import these "partially fulfilled" orders from Shopify into Shipping Easy. They show up in Shopify but not in Shipping Easy.
Here is Shopify's documentation on the issue: https://help.shopify.com/en/manual/orders/fulfillment/fulfilling-orders/fulfill-your-own#fulfillment-holds
Here is additional documentation from an app we use (One Click Upsell): https://help.zipify.com/en/articles/4600485-oneclickupsell-native-working-with-shopify-orders#3rd-party-fulfillment