I found my local post office the most convenient place to ship from. Luckily, most jewelry is small and light. So, I determined between the packaging (one bubble envelope and a little tissue) and posting, most items cost about one dollar to mail. Therefor, I charged my customers an extra dollar for this purpose.
You would need to determine your own shipping costs and develop your own system. Though I charged my customers for the cost of the postage and packaging, if you have to make extra trips to the post office or other carriers, you may want to factor in your time or gas too. Its up to you to decide which works best, but you need to make this decision before selling because you should inform your customers of any extra expenses they will be charged for.