Almost as bad as someone walking into a brick and mortar store and then going to some big website to actually buy the TV. No fair indeed.
If I'm at the store and i can put something in my cart and buy it then I'm not going to order it online and wait for delivery to save $5. Also delivery is extremely unreliable these days...
From watching other people at stores, it's maybe 15%? For goods that need to be installed or carefully handled, it's less common. Headphones? Pricecheck. Mechanical Keyboard? Pricecheck. Washer and Dryer? Most people pricecheck even though it seems like something they wouldn't. Turns out that most people shop around for those, so internet pricechecking is part of it. Food items that seem too expensive? Pricecheck. You can save a bunch on dry/bottled goods. Amazon Prime feels like a Costco membership. You have to make sure you use it, to justify it.