We have released support for multiple packages. This will work for UPS Online API and FedEx Web Services.
There is an extra link "extra packages". Click on that and you can add more packages to the same order.
You are limited to have the same carrier on those extra packages.
When you print label for the order, there will be multiple label printed. One per package. These packages have a single "master tracking number" that will be confirmed back to source.
If you reprint label for the order, all package labels will be reprinted. If you cancel the label, all packages will be cancelled.