App works well, only problem is inventory isnt always updated. They definitely need to fix that because you have to call them to figure it out
We've been using Shipmonk (and their associated Shopify app) for more than a year now.
The onboarding process is fairly easy and they help you connect your store to their fulfillment center, and map the SKUs to their internal inventory codes. You can also do it yourself manually if you create product bundles, for instance, or they will do it for you if you have a lot of them, just by sending them a CSV.
From that moment on, Shipmonk automatically captures the orders from Shopify, processes them, and then inserts the tracking number once the order has been fulfilled.
So all in all, it is a very straightforward process and the only work you need to do is check their interface daily to verify no orders are "stuck" because the buyer entered an incorret address, or you have run out of stock of any item (which you shouldn't, as they provide a forecast on how many days of inventory you have left at the current sales rate.
All in all, very satisfied with their service and the timely delivery of the orders to my customers.