The charge is stored in the metal part of the jar. There must be some metal to hold the charge. The reason you get a shock when you are touching both the base and the top is that you are creating a circuit, whereas when you are only touching one, then there is no circuit. This may not be a complete circuit, but the jar is really a capacitor, with metal on the inside and some metal on the outside. The two metallic things are the two sides of the capacitor separated by the wall of the jar. They naturally want to have the same charge on both sides. After you charge the jar, one side has more charge than the other. So if you touch both, the electrons race around through you to achieve balance and you feel the shock.