Yes, sometimes its a minimum $ value to score the free shipping, sometimes weight is a factor.. all depends on the store & alignment of celestial bodies.
As was posted elsewhere, the BB granite plates seem to get scooped in Calgary. Always on sale, never in stock. they may as well say free if you cant get one. I saw them available in other stores but they don't seem to have an inter-store (free) item transfer like other chain stores offer. So first they said free ship, then they retracted & said too heavy to ship so will cost X. Seems like they only honor the sale price of in-stock. Eventually I became fatigued of the runaround & I will just source elsewhere.