^ its authentic. no worries.
A "thank you card" from any store can't prove any authenticity in any way.
For your own inspection, few things you can check, the size tag.. make sure it is printed with the correct model code, size with width (EE, E or D) and made in USA (yes for almost all rw).
Next will be the shoe box, normally the label sticker outside has all the details, make sure the label looks "original" and of course, correct details.
Lastly, the quality. Fake one can be easily be identified when comparing with the authentic ones. The vibram soles, weight of the shoes, stitchings and so on.
Hope it helps.