When I last tried it the free delivery for an over £20 spend wasn't automatic. If you ordered items which added up to more than £20 you had to deliberately combine the order and deliberately claim the free delivery - you weren't offered it as a default option.
And their delivery charges are high. Seems to be about £2 if the item will go through the letterbox and £4 or so if it won't.
Royal Mail is much less. And the postie knows where my place is and how to get into the flats. And I know roughly what time they will call if not the day - so I don't have to hang around waiting for a delivery.
I haven't ordered anything from Amazon since 2017, partly because of the hassle.