I would text him back saying “Thanks Ron, go ahead and mail it to me, if you wouldn’t mind”
This kills 2 birds with 1 stone. You don’t have to see him or talk to him, and it will call him out of the fact that the necklace is not yours (if you’re 100% positive that it isn’t).
If he knows it’s not your necklace, then he’s just doing it to make you feel jealous. Even if you do feel jealous (just a tiny bit) don’t let him know that. Let him send you the other girl’s necklace. Because then he’ll feel like a dope for even attempting this. And likely, the other girl (who’s necklace it really is) will be pissed at him too.
He most likely won’t send you the necklace, because it’s not yours. But if he does, because he’s too dim to realize that it was someone else’s, then take it and keep it if you like it, or get rid of it at your leisure.
If he keeps on texting you, in what seems like an attempt to get your attention, just tell him that you feel it’s better if you dis-continued having contact with him so that you both can move on.