So how about this:

Prerequisite: Warlock level 3
Bonuses:
1. Increase Charisma score by 1, to a maximum of 20.
2. Gain one invocation of choice from those available to the warlock and for which you qualify.

OR
Prerequisite: Warlock level 3
Bonuses:
1. Gain two invocations of choice from those available to the warlock and for which you qualify from the following list of pairs:
Armor of Shadows + One With Shadows, Eldritch Sight + Eyes of the Rune Keeper, Fiendish Vigor + Gift of the Everliving Ones, Mask of Many Faces + Misty Visions, etc.

How does that look?