Just FYI, FedEx API is a disaster. To ship multiple packages within a shipment you first have to create a shipment with one package and then add packages to that shipment ensuring that you then set the shipment_id property in each of those packages (and if I recall, you need to know number of packages when creating the shipment). This is but one small example of how screwed up their API is. Then there's the whole issue of printing labels to a thermal (or other) printer from a web browser that meets their specifications.
This is old feedback (a few years at least) since I last looked into it.
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