Ask them how you were meant to find out they wern't fit for purpose if you couldnt try them out.
Fit for purpose, quote that to her and refuse to leave, make a racket, stand your ground, get some leaflets printed explaining EXACTLY what has happened (no lies, untruths or exaggerations) and stand outside the shop and distribute them to custards going into the store, wont be long before they give you your dosh back.
Only give them the two smallest of the three sections, theyre too thick to realise theres meant to be a third, you can use that third section for getting over gates, low walls etc on awkward properties.