I would think that the value for customs/duty purposes would be zero or close to it, as the pack is defective. I'd declare it at $0 and label it something like "defective battery pack".

You might also ask the Gemini people if they've considered setting up a US shipping address just across the border (say, in Blaine, WA). I know they're a small outfit, but if the US is going to be their major market, it would probably help. The idea is that a storefront receives packages for them at a US address, then trucks them across the border to the company in Canada, eliminating the customs issue for US senders. This can also be done in reverse for shipments to US customers.

I've done business with Canadian companies with such setups, although they were Vancouver-area based (less than an hour from the border), and Victoria involves an additional ferry ride to get to/from the island.