Bubble envelopes would cost more to ship because of their size and bulk. I would suggest using something like these rigid card holders
inside a normal envelope. I would suggest printing an invoice and wrapping the card holder with it for shipping. If you do it right, it should be well secured and cost $1 +/- to ship.
On eBay, you can earn PowerSeller discounts if you meet certain requirements like a certain amount of sales. It's a bit more complicated, but if you want to know more, you can refer to this