I think the international rates look fine for 10 oz packages but I've never shipped an individual piece of jewelry that weighed more than 3 oz (including a 3x3 inch cotton-filled jewelry box and bubble mailer).
Using Etsy labels, Domestic First Class postage for up to 3 oz is $1.93 including Delivery Confirmation, so I think $6.50 is a bit of overkill for those.
If you don't have a scale either get one (less than $30 including shipping on Ebay or Amazon--well worth the small investment since it makes shipping from home easy) or take a few items--lightest, heaviest, middle-weight, to the PO in the packaging (as if you are going to mail them) and ask the clerk to weigh each for you and set your rates according to the actual weights the clerk tells you.
A side note, in case you hadn't noticed: International First Class goes by the ounce up to 8 ounces, then they change every 4 ounces & parcel postage rates start at 2 oz.
(2 oz, 3 oz, 4 oz, 5 oz, 6 oz, 7 oz, 8 oz, 12 oz, 16 oz, 20 oz....)
A 1 oz or 2 oz package costs the same ($6.16 online postage) anywhere in the world, after 2 oz the rates change based on the country it's going to and the weight of the package.
And a 9, 10, 11 or 12 oz package costs the same to lets say the UK, a 13 oz, 14 oz,15 oz, or 16 oz the same...