I am really sorry to read the terrible situation you have to face. Who would think that trying to buy some cross stitch threads at a reasonable price would lead to such a cost.
Have you been in contact with any consumer protection association?
As tenants we are in contact with one who is doing stunning defense for us against our abusive landlord for the affordable price of their annual cotisation.
Maybe you want to check the following websites and see if you can get any help:
Associations de défense des consommateurs - Service-public.fr
Associations de consommateurs / Outils - Le site du magazine 60 millions de Consommateurs
Association de consommateurs - UFC Que Choisir
Association Lo Lagrange pour la Dfense des Consommateurs (ALLDC)
I send you my sympathy and I sincerly hope that soon enough you will be able to put this whole horrible story behind you.