Can BYOB support my fulfillment app?

It depends on whether your fulfillment app supports order edit by Shopify and how it syncs the orders. We follow the discount & shipping calculation written on the official docs - edit order

If you want to know if your app is ok, please send us which fulfillment app you used, e.g. ShipStation, StarShipIt, etc. 

For StarShipIt, the weight is doubled after syncing BYOB orders from Shopify. This is a known issue due to the order format and there is no feasible solution yet. Users have to do manual adjustments just like adjusting discounts.