The Amazon delivery charges are a nightmare, and like Edders I don't order enough to justify Prime, and have no other real reason to join Prime.
So generally I try to only order stuff worth more than £35 to get the free delivery, or try to wait and group smaller items together so they come to more than £35.
But even then it's not straightforward, and sometimes free delivery is available on cheaper items, and sometimes it isn't, and it often depends how precisely you group or order these things. Last time I ordered something (some keyfob batteries for the Octavia

and some hoover bags) when I grouped them together they were saying that I needed to exceed the £35 for free delivery, but if I ordered them separately they were free delivery anyway, and actually came in the same package
Another one last year year was replacement LED strips for my rooflight, which were £7.99 or so, and thought I'd wait until I needed something else before ordering to get the free delivery rather than paying £3 or so delivery on a small item. Then after waiting months I suddenly realised that if I got them delivered to the locker in town I usually use (I'm reasonably remote at home for delivery purposes) then it would be free delivery anyway
And the locker thing is another factor as regards delivery charges - sometimes it seems to help if I specify the locker in town for the delivery rather than where I live in the middle of nowhere, and sometimes it doesn't.
But, of course, it tends to help if the items are marked 'dispatched by Amazon' rather than delivered directly from a third party, and also if the items are marked Prime, although I'm not actually a Prime member

Another thing is that items 'dispatched by Amazon' tend to arrive a lot sooner than the maximum specified even by standard delivery, so hardly worth paying for next-day delivery or whatever if it only in reality takes one extra day via standard delivery (although obviously there's no guarantee in that regard).
Whereas the items despatched by third parties can often take a lot longer to arrive, even if they're marked free delivery
Apart from all that, it's generally all quite straightforward
