So it looks like it was $385, but still too much, so they may be charging $x for short locate, and $y for short/borrow interest rate, but neither of these are known right now. So ask them for specific numbers.
IB shows short borrow rate as 700%, so 700%/365 *7 = 13.4%. Th stock dropped by more than 50% overnight, so there is still room to make money. Even at 3 days (over a weekend) it would come to 700%/365 *7 = 17.25%.
But we don't know whether different brokers may be able to use different borrow rates. And again the locate fee.
And I don't think that 3-day settlement applies here (as someone else pointed as possibility) because you closed the position next day at the price of the next day, so you no longer had a position. If short borrow rate applied to the 3-day settlement then you'd still have a position and they would need to close your position 3 days later at the unknown price of the stock 3-days later. This wouldn't make any sense.
Just ask TradeZero for specifics as a starting point. Though I remember looking at their website months ago and noticing the short borrow rate being unacceptable.
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