Hi Neil,
Thanks for your message,
If your rental company is charging you a lot for the snow chains, maybe it would be better for you to buy them as you can find them easily in car workshops, gas stations and even supermarkets, and they will cost you around 60€.
However, I would recommend you if possible to get them before crossing the border as if the police stops you in Andorra and you don't have them you can get fined.
Please, let me know if you have any questions,
Kind regards,
Esther