Easy fix...they should refund you twice...once for the purchase, next for the error...seems just a simple error that the sales person either did not notice or did not pay attention to.
If not and they are not sympathetic perhaps a mention to them that they have just stolen from you and a call to the police may be the next step if it is not immediately remedied is necessary to light a fire beneath them.
Lesson learned regarding long distance/time purchases...not knowing the shop's return policy you are lucky to have been refunded a dime...what are they going to do with a used saddle?