If your jewellery doesn’t weigh very much then you’ll probably find that it costs the same to send two or three pieces as it does to send one piece. This definitely applies within the UK as the weight/price band for 1st and 2nd class goes up to 1 or 2 kg. Globally the weight/price bands go from 0-100g, then up in 150g - 250g increments (so 100-250g, 250-500g etc).
It might just be easier to offer free postage on all additional items … it’s certainly easier to set up anyway.
I started doing it a couple of years ago and it’s made a huge difference to the number of items people shopping from outside the UK buy at once. I very rarely have to pay more for postage than what the customer has paid and in the instances I do pay more it’s because the customer has bought a lot of stuff so I certainly don’t mind paying out a couple of extra £s in postage.