When you sell shares, cost basis is reported to the IRS by the brokerage. The parent comment suggests that RH always reports the FIFO cost basis, i.e. price of the first shares you bought as opposed to the last shares. There are places on tax forms where you can adjust your cost basis up or down from the one reported by your brokerage, but (without doing any research into it) I doubt that "I don't want to use my brokerage's cost basis method" is a valid reason to use those.