There will be 100 SKUs and 200 shipments daily via USPS First Class mail. All shipments are under 3 lbs; 80% of shipments are under 1 lb.
I'm curious about which software we should use to generate labels based on orders placed from a Shopify store and which software we should use to track inventory as orders are fulfilled, provide tracking information to customers, etc.
Try the guys at theprintful.com they have made a good niche business providing the services you seem to be needing. Best of luck, learn from them and keep asking questions ;)
Shipstation is best. It can also extend outside of Shopify to other marketplaces. With that many orders under a pound you may also want to consider DHL eC. They can often beat First Class Mail rates for very small packages.
I would definitely recommend ShipStation for your intended application. ShipStation can cover all your bases. It can quickly assign appropriate carrier / service levels to orders through automation rules, allocate and track inventory, and send customized tracking update emails to your customers. Once you're set up in ShipStation, they can introduce you to high-volume shipping services such as DHL eCommerce which will save you money on shipping.
Let me know if you have any questions!