When you go to Settings > General in the app you will three sections for Products, Fulfillment Services, and Shipping Zones. Each one has a Sync button.
Syncing always goes in a one way direction from your shop -> the app.
Sync Products
Whenever you add products, or edit products in your Shopify admin, and then open the app, it will ask you to Sync products. Most of the data we sync is to keep the product catalog updated in the group assignment page.
You DO NOT need to open the app and sync products each time to make changes to products, however, you should sync products each time you open the app before making edits to shipping rates.
You SHOULD open the app and sync products anytime after you've added new products that should belong to a non-default product group, or if you've manually adjusted a product's fulfillment service.
Sync Fulfillment Services
You should sync fulfillment services anytime you have made changes to manually created custom fulfillment services.
If you have created your fulfillment service through the app, then you don't need to worry about this.
Sync Shipping Zones
You need to sync shipping zone anytime you've made changes to your shipping zones. This one is super important!