The Costa Fantail's I bought off eBay are 100% authentic. I even went by the local Academy Sports & compared them to a pair they had in stock & they were the same in every way, including box, paperwork & case...
That being said, I'm sure their are counterfeit Costa's out there, so buyer beware...
Here's some tips on how to spot fake Costa's..
1) Turn your sunglasses from side to side while looking at lenses.One of the key features of Costa Del Mar sunglasses is that they only offer polarized lenses.Costa Del Mar polarized lenses will appear black when turned from side to side. See https://www.youtube.com/watch?v=7KSWQAceo6g
2) Watch inside temples of the sunglasses. Laurie Fontenot, Costa Del Mar representatives, saying that all the company’s sunglasses has a “stock unit numbers” and the name of the style sheet imprinted in this area.They will also wear the Costa Del Mar the logo(The wave styled C) and the name of the manufacturing site inside the temple.
3) Check the lenses of the sunglasses.Costa Del Mar sunglasses have practically glare-free lens called “580.”Authentic sunglasses will have many “580P” etched on the upper outside corner of the right lens on PLASTIC lenses & just "580" on GLASS lenses.
4) Look at the hinges in sunglasses.Only stainless steel hinges are used in all the Costa Del Mar eyewear.Fakes will often have poorly-adjusted hinges.
5) Check the overall performance of your sunglasses.Any scratches or imperfections in the lenses or finish are indicators that the glasses are fake.
6) Take photographs of real Costa Del Mar sunglasses with you and compare them with the sunglasses.The photographs can help you detect subtle differences in the knock-offs.
7) Check the weight.The international anti-counterfeit Coalition says most counterfeits are utilizing cheaper materials making them much lighter in weight than the originals.