Me: Sometimes I'll buy multiple books from separate publishers, but at the same time. I'll add books to the cart week to week, the carts stay as-is in my current browsing session, then I'll visit each cart at the end of the month to place an order. Usually, I'll get a few boxes from different publishers around the same time, depending on shipping speed.
1. I only buy (and read) a book that is at least 10 years old. For me, 10 years is enough for other people to figure out if a book is worth to read. 95% of all books that are today's bestsellers turn out not to age well. Because none of the books are new, I can usually buy them used.
2. I'm only buying it if I'm pretty sure I'm going to read it. I always read the digital preview and sometimes (if available) I download the digital version which I read for a while. I want to know if I'm going to read the book before buying it.
This is a list I keep separate from any one store, and I reorder it until it settles into a pretty good order. I have been really disappointed by cart functionality and have even lost some items etc.
I do tend to order in batches but mostly because I wait for sales and they usually cover more than one book I want.
Slow shipping is OK with me.
Some of the smaller sellers are fun. I like that Wayne's Books throws in a cool little business card. I like that Brett at Pigames.net remasters vintage game books.
Now and then I check some of the books reviews on goodreads (the text reviews) and or a couple of them or, if I want to treat myself, 10 or 20 of them. To be, receiving 20 books is the definition of happiness, and one of the few things I will happily splurge on.
Delivery times are normally irrelevant. I try to keep a steady trickle of vintage books entering the house. Christmas every week! Since most are in the public domain and available for free online, I often skim the online copy to determine whether a physical vintage book warrants the shelf space.
Technical books tend to be more expensive, so I usually buy them singly and try to order direct if possible. If I'm doing research on a specific topic I might buy many, but I usually do one at a time. I've found that having too many books in my "to-read" stack gives me a kind of anxiety that stops me from making progress.
I don't remember ever leaving a cart open after I left a site.
Interval is sporadic. If I think it is urgent, then order it immediately, else keep in Amazon wish list and buy when there is a good discount or during sale. If physical copy of the book is expensive, then I consider kindle edition if kindle version is much cheaper.
This is especially convenient since I only read on a Kindle. So shipment is not a thing I have to think about.
If it is cheap enough, I usually buy.
If it is available but not cheap, I typically let go because there will always be another book I want just as much.