Sometimes, voltages collide. When you have parts that expect/speak at different voltage levels, you have to build a mediator to smooth things out. This SparkFun BOB-10403 uses PCA9306 two-channel bidirectional voltage-level translator on a nice, small breakout board.
The Dimension Engineering 5V switching regulator is a tight & tidy voltage regulator that offers great performance at an excellent price. Great for extending battery life!