faucet pipe too short

Get flexible hoses. They come in various lengths, so you should be able to find some that are just a tad longer than the space you have between the fixture and the cut off valves. The type with the steel braided covers seem be be preferred by most. You may have to get adapter fittings to match up.

The extension tubes you have can be cut to size with a tubing cutter, but stainless steel braided hoses is the easier solution for a DIYer.

If your faucet supply tubes are straight copper, you'll probably need a couple of 3/8" brass compression fittings to match up with the stainless steel flex lines attached to your shut-off valves.
You don't need pipe dope or teflon tape with these fittings or lines. The flex lines come with built-in rubber seals, and the compression fittings seal themselves with brass ring ferrules.
Good Luck!
Mike