Here's my example that it pays to compare. Few months back, knowing I wouldn't see a loved one for their birthday, I ordered a cake from a company occasionally featured on QVC with products that sell out. The company's own website had a 25% off sale, with a shipping fee of $7.95. Worked out to be slightly cheaper than QVC.
Now for a July birthday I went to order the same product. This time the company did not have a sale, the shipping was $7.95 plus an additional $7.95 "hot weather" shipping fee tacked on. QVC had the product for the same base price and as per usual zero shipping cost on food items. This time it was cheaper to buy from QVC.