We use 2 different fulfillment services. Can we pick and choose which orders FBA Shipping fulfills?
Can we choose a fulfillment service for our orders after they are placed?
Unfortunately, the short answer is no. Shopify requires that each of your products are assigned a fulfillment method during initial setup. And then that products are assigned the correct shipping rate within your shipping profiles.
So, in order for our app to fulfill orders the products would need to be set to FBA Shipping and in the FBA Shipping profile before the customers place orders. There is not a way to change the fulfillment service after the order has been placed.