... how many purchases they are likely to make at the same time.
Agreed! 1 client, 2 sites/stores: one as Add to Cart and 1 as Order - for just this reason.
These days I just give them what they want, but as an option, so I don't have to change the bloody code back again.
I understand! Seen the length of the Pro_News Config screen recently?
But more and more recently I've found I can meet user requirements through specialised Pro_News templates! Really helps, as the base code (Pro_News) doesn't have to be retested each time. (All learnt through reading reDesign templates - php includes - and Storez templates - complex IF conditions!)