Of course qty makes a difference but
I use Staples, Walmart and Lowes for the majority of my shipments. They are my box warehouse
USPS priority which are free, and knowing the A B regional system/rules/rates can be a cost savings
I also keep large flat "stock" from incoming shipments for box reinforcement and custom size boxes.
Almost as much as the box are the bubble wrap, peanuts, paper and foam pieces to protect the contents(95% recycled, every incoming shipment makes a donation to the supply)
A semi auto tape dispenser for box taping makes for a more secure box with a tape gun with clear tape for the label (buying in qty on Ebay the cheapest)