We’ve found that FedEx provides the easiest path to ship batteries. Their website has a search tool to find FedEx locations that can accept batteries can be found good shipments, go to https://www.fedex.com/locate > More... > Dangerous Goods Shipping > and Search in your area.
If you’d like to pack the batteries yourself, guidelines can be found in the UPS guide to safely packing batteries. Refer to the Lithium batteries section (SL batteries are classified UN3481). These guidelines are universal across shipping providers.