The rigid mailers are thin enough (if packaged properly) to be sent as a large letter, so postage would be according to weight - $1.20 for up to 125gm, $1.80 for 126-250gm etc.

If you do a search using terms like "flat mailing box", "rigid envelope", you should be able to find the ones I'm referring to - there a 18cm x 10cm size that is good for small jewellery items.