I think it's fair. If you order it online and hire someone to install it, and it goes bad, it may be warranty but you still get to pay the guy to do the job the second time. I imagine from the local guy, if there's a problem, he gets to replace it for free and battle the manufacturer for reimbursement. That's where the markup comes in.
I'm a mechanic by trade, and I've run into similar scenarios before. Here's what happens:
Example one. Guy gets towed in with a dead battery. Find a bad alternator. Sell alternator and 50% markup. Charge for time to install. Two weeks later, battery light on dash is on. Parts store is happy to warranty the part, but I install it for free. There went my 50% markup...
Example two. Ggal comes in with a rattle. It's her water pump. I give an estimate. She says let me think about it. Two hours later she shows up with a pump from the cheapo parts store. I start the job then discover it's the wrong pump. I call her. She calls store. Store calls me. I end up measuring the hub diameter on the old pump. She comes back, takes pump, leaves me waiting with a car dismantled on my rack, returns with right pump. I install it. I got nothing extra for ordering the right part for her nor my downtime. Three months later, she's back with a coolant leak. Water pump is leaking - she can see it plain as day! I tell her I'll be happy to replace it again for the same price as last time. There was nothing wrong with the installation - and I didn't sell her the part. There's nothing for me to warranty. And after more wasted time arguing, a couple more phone calls, threats to report me to the Bureau of Auto Repair, I decided I will never again install parts they bring in.
The time to question the price is before work starts.