How much are you charging customers for shipping? And do you have a threshold where they get free shipping after spending a certain amount?
You might want to talk to FedEx about their One Rate program. It’s flat rate, 2-day, under 50 pounds. It cut my shipping costs in half and allowed my customers to pay less for faster service.
Use Clickship. I ship 20”³ and 12x12x24 boxes mostly, and it’s way cheaper compared to using my own UPS/FedEx account.