I was thinking exactly this. The mentality of "geez man you've already got the bulbs just sitting right there" overlooks the preparation it took for the business to be, well, prepared.I think you missed some other time like them doing the check to find anything wrong. Include that time in and also the time to write up the bill and process the payment. Then there's also the cost of having inventory on hand, it costs money to maintain as you have to order parts as they're used up and they're not making any money while just sitting on the shelf waiting to be used. I wouldn't say she was ripped off, but it was certainly no bargain, but probably the going rate at a garage. It's like places that have $100 oil changes, you could buy the materials yourself for $25 and DIY but it's not a total rip off, you can certainly shop around and find cheaper places.
It's always felt a bit like the kid who wants to copy your homework in school: "Geez man, you already did it, why can't I just copy it??" Don't wanna pay?? Then do your own homework, ie stock or source the bulbs yourself.