Like so many have mentioned here I am irked with each and every book having to go through as individual purchase. Same when using the Kindle store, while I can buy via store credit/gift voucher to avoid a lot of smaller charges going through onto a card individually it can still be annoying.
More than anything it is easy to loose track of credit balance as every book is a separate charge, keeping on top of the budget is easier if I can group books together. So I don't accidentally over step my budget and begin charging to my bank without meaning to. This happened when my apple account was hacked last year.
The hacker used up my credit and then charged over £100 more to my bank even though I had removed my card from my account more than a year beforehand and it didn't trigger any warning bells despite me never spending more than four or five pounds at any time in the years previously let alone a total of over £150 total within 48 hour time frame.
I would have caught it sooner but apple don't send out confirmation emails immediately and often it takes until 72 hours after purchase so I didn't know anything had been bought/downloaded using my account until after the hacker had went on a big spending spree.
Luckily enough both Amazon and Kobo confirm almost instantly so I can catch any possible fraud quickly and contact merchant before a hacker can build up a large bill on my name so that's one things I don't have to worry about. But that's beside the point and sorry for going a bit

for a moment.
Ordering by basket and not individual books helps me keep track more easily my book budget. Also I know some banks either levy charges for each transaction including those who pay in a different currency that their own, or like mentioned but I haven't so far come across myself become flagged as suspicious causing your card/account to become unusable while you verify that it was you using your card not a hacker/fraudster. Must be a real pain for those it happens to.
But to cut to the chase I would like to combine book purchases together in a basket for putting through in one order for more than one reason.