It was my first time preordering a bag, but this was my experience when I reached out to a SA yesterday to see if I could preorder the beige iridescent mini rectangular (which btw is apparently in calfskin not lambskin!). The reason why I even reached out to her is because said she sent out a blast that she was only taking preorders within a 48 hour window, and that confirmed preorders would require a 50% deposit. My preorder isn't confirmed though - so I'm waiting on confirmation which then the 50% deposit would be charged to my cc. I also saw some personal shoppers on Instagram saying that this 48 hour window is the period where boutiques submit their buy orders to Chanel corporate for what they want as their stock for any particular collection. That's a first I've heard of this, but again, my first experience preordering was only yesterday. My NM SA stated that she hasn't gotten the books yet for 21SS.
I personally think I was just lucky to have been notified by her that she was taking preorders for 21SS, otherwise I wouldn't have known about preordering as I too usually just call around (or precovid I would actually go into the boutique to see what's in stock lol) and ask if something is in stock.