Currently "Sodium Battery Cell" is priced at US$50–70/kWh on the Shanghai Metals Market, which gives every appearance of being a real thing: https://www.metal.com/en/markets/43. They list LFP batteries currently at US$38–52/kWh: https://www.metal.com/en/markets/42

I think you may be dramatically overestimating how much container shipping costs.