You can print the Shipping Label that Etsy offers, it is right there on the order. It is easy and convenient, as the shipping label also works as Customs declaration. I use it and am very happy with it. Now, you need to know the weight of your parcel (I have a scale for that).
If it is not very heavy, Flat rate is not a good idea, as it is more expensive than First Class and there is no tracking options for Priority Mail either. Tracking is only available with Express or Global for international shipping. One more important thing: jewelry might be tricky when it comes to customs, so be aware of that and act accordingly. When you say jewelry, they may think you are talking about diamonds or gold or stuff like that, which might not be allowed to ship. Make sure is all right.