Unfortunately 99% of the shipping software is written for PC and not for Mac, the engine we use “Shiprush” only can process shipments on a pc, I spoke to the developers and there are really no plans to develop an engine to run on a Mac.
However saying all that, you can do almost everything on xTuple on any platform and then just have a workstation running windows (it can be a Mac running windows with VMware) to do the final step of printing labels and submitting shipment to carrier.
ShipIt uses ShipRush® to process all shipments.