Best Shipping Method?

USPS in a bubble mailer with tracking. Put the cards in some sleeves and toploaders.

Put the card in a soft plastic sleeve, and then put the sleeved card inside a top loader. Make sure the flap that you inserted the card into the soft sleeve is at the bottom of the top loader, to prevent water from being able to touch the card even if it was submerged in some accident. Taping the opening of the top loader is another safety measure you can take. The shipping envelope used should be a bubble mailer (an envelope lined with bubble wrap).

As far as shipping services, USPS First Class is the standard method. It's the cheapest method that provides tracking. Your tracking number will be on the receipt, and you'll then contact your buyer and provide it to him - you don't need to put the spaces in between the numbers, it should read a a single long number with no breaks in-between.

The cost of the USPS First Class service is about $3.50 in person for packages between 1 oz and 4 oz. USPS First Class packages have a weight limit of 13 oz, then you'll have to ship with USPS Priority Mail instead. On eBay it's about 25% cheaper if you sold through there, and you can directly print shipping labels. But you'll then be dealing with eBay (10%) and PayPal (2.9% + $0.30) fees, so keep that in mind.

This is the method that I personally use, but there is also the option of simply using a plain white envelope with USPS standard shipping. I personally only sell cards <$2 this way on my second eBay account (and typically only for Magic the Gathering), and there's risk with this. First off, the method is the same: Card -> Soft Sleeve -> Top Loader -> Tape. But instead of a bubble mailer you use a plain white envelope.  I put half a sheet of printer paper (standard 8.5 x 11) folded around the top loader, and place it in the envelope.  From there it's just like mailing any other letter, put your return address, buyer's address, and a stamp on it.  A single stamp will pay for 1.0 oz in weight, which will allow up to four cards, the sheet of paper, the top loader, the soft sleeve, and the envelope.  Additional weight added will require an additional stamp, though you can ask the clerk at your post office to weigh it for you and then put the second stamp on there after they tell you that one isn't enough.

Keep in mind that the second method is much riskier as you are not getting a tracking number, and your buyer will be in the dark about its whereabouts until it actually arrives to them.  Expect problems to come up using the plain white envelope method. Expect people to ask for their money back doing that.