Voucher not usable

I purchased a voucher online from Crocs SA for Christmas. I gifted it to my mom. When we got to the Crocs store this week, we found out the voucher can only be used online! They don't have the size available that my mom wants online so I ended up buying the shoes cash at the store. I want a refund for my voucher as I wasn't notified anywhere of restrictions wrt using the voucher. I'm very unhappy!