Personally, the only webshop where I actually need so many returns is Amazon itself.

Some of that is intrinsic to a good chunk of what they're selling (poorly described imports from the Chinese market). I just ordered an assortment of cabling for shuffling PCIe signals around. Apparently there are a few different "standards" for pinouts on SFF-8643 and SFF-8654, but nobody actually describes them in their product listings! So I'll see which ones work with what I already have, and with each other, and the rest can go back. Sellers could have avoided that by doing that work once and describing their items, but they'd rather I do it instead. So when I do it, I don't feel bad about the returns.

But a lot of it is the dark patterns Amazon themselves choose to play. Nonstop pricing games make me push my actual purchasing decisions down the line. If something is on sale at a great price and I'm in the market, I'll just hit buy to lock in that price. I'll then try to do some research before it ships, but in general I've got 30+ days to make the actual purchase decision. This is what the seller must want, right? Or the same pricing games but after a purchase - if the price on something significantly drops, the answer I've gotten from CS is to buy a new one and return the old one. So if I already have other stuff I need to make a return trip for, that's what I do!

Or when I'm in the middle of building/repairing something, especially when I don't have a "trial" of "prime" (aka sunk cost fallacy), I'll order a few different types of items I think I might need. This way when they show up in a week+ (Slow, Spiteful Shipping) and I'm back focusing on the project, there is less chance of "oh I really needed this other thing" and having to wait another round trip.

With other web stores I just don't feel these type of dark pattern dynamics. Whereas Amazon is practically begging people to call their bluffs. So I call them.