How do you order shoes online and know that they'll fit?
The trick is to order brands of shoes that you know from experience will fit.
Compared to what others have said here, I buy LOTS of stuff online. Always when I get free shipping, by ordering a qualifying amount.
I buy my loose leaf teas from Arbor teas. I buy Wally's Celsestial Seasonings Blueberry tea from Amazon.
Also from Amazon, paper plates made from sugar can fiber (they come in cases of 500) I used them not only to eat, but as cutting boards when I don't want to have to wash the cutting boards I have. Years ago I bought a vacuum from them, since I got free shipping. Let's see, also shoes (see above) I buy a particular Champion sports bra I use as a bathing suit top, and recently bought swim bottoms from a swim outlet online.
At this time of year I would also search online for spray sunscreen to stock up for next summer.
I also buy a case at a time of 12 oz paper coffee cups. That lasts me 2 or 3 years. Right now I'm using surplus starbucks cups I got really cheap.
I get my 3 month supplies on perscription drugs online, and I'll purchase supplements if it's the right brand and price.
In the past I've bought things like shampoos, gel and mousse and hairspray online.
2 years ago I got two sets of cushions for my outdoor gliders.
I used to buy certain clothing items, but since I'm no longer working full time, I don't have that need anymore re work attire, spanx (I've got a dozen pairs in my drawer that I do wear under certain pants) and things like that.
Basically anything I use regularly, if I can get enough for free shipping, the price is right, and it's something I can store.
I'm sure I could think of a lot more things, but it's past midnight and I'm going to bed.