Fren, there are two native North American Yellow Ladyslipper Orchids. Cypripedium calceolus (Yellow Lady Slipper) and Cypripedium calceolus parviflorum. For reference I have attached two thums illustrating each:
Cypripedium calceolus
Cypripedium calceolus parviflorum
As you can tell, there is not a lot of difference between the two other than the Small Yellow Lady Slipper is (obviously) smaller and grows under differing conditions. I checked C. parviflorum and it is syn. with C. calceolus parviflorum. My feeling is the vendor
may be correct as well as you are probably correct.