IMHO, one can never have too many O-rings so I routinely restock my assortment by purchasing a variety of sizes. I also use them as elastic bands. My wife's handbags have O-rings on the strap to keep the loop under control. She even uses an O-ring as a discrete belt loop. Better than duck tape!
I live in an old house (early 1970's), so many of my fixtures are some kind of generic ones sold to builders who were building tracts on the cheap. That said, they leak periodically and I need to replace o-rings. Each time it is a big project to find parts. No, it is not as easy as going to the local hardware store ... the big DIY chains drove the real hardware stores out of business. We had one just down the street ... it had been around since the homes here were built and so the owner knew every replacement item needed. (They eventually became part of the ACE franchise then went out of business to retire to Florida.)
The problem with going to Home Depot or Lowes is that you can't get the kind of advice that you could get from the local hardware store. The people working there are usually kids who have no clue. You are left to rooting around in racks of sealed plastic bags of parts made in China. Unless you start ripping bags open it is hard to see what fits or does not.
The main reason to buy online is to get a large collection of things in the hope that one might fit when needed and you don't have to shut the water off to the house for hours while you drive from place to place looking for the right fit. And as to shipping .... how much are you willing to spend in gas driving around the county?
Now for my question: you mention that you restock with a variety of sizes. What does your stock consist of? I'd like to get a stock of useful parts so that when I need it I will have it. I'm tempted to just buy one of those boxes of 500 or so o-rings but am wondering if that is the way to go. Some of the comments are negative citing cheap Chinese manufacturer. (Rings not holding shape and so on.) Is there a "brand" you find to be of quality or have the Chinese ruined yet another part of our life?
Any advice or suggestions will be appreciated. Thanks.