Heads up, investors: A provision in the Senate's would take away your ability to pick and choose which stock shares you unload when you go to sell.

If the move is included in final legislation, investors would pay an estimated $2.7 billion more in taxes over a decade, according to the Joint Committee on Taxation.

"It's a good move as far as tax reform goes because it simplifies taxes, but it will leave investors with fewer choices when they sell stock, and they'll pay more in taxes," said Steve Rosenthal, a senior fellow at the Urban-Brookings Tax Policy Center.

Under current law, investors with taxable brokerage accounts have a couple of choices when putting in a sell order for stock shares: They can tell their broker to sell the oldest shares first (the first-in-first-out or FIFO method) or direct the sale of shares that were purchased on a particular date (specific identification method).

For holdings in a taxable mutual fund, investors get a third choice that could be retained under the Senate bill: using the average costs of the shares.