This ability isn't a mana ability. It uses the stack.

Only the first main phase in a turn is the "precombat main phase," even if additional main phases are generated by some effect.

If the enchanted permanent's mana cost is , this Aura's controller adds . If its mana cost is , this Aura's controller adds . If its mana cost is , this Aura's controller chooses whether to add , , or . The choice may be different each time the ability resolves.

You get the mana whether you want it or not. If you don't spend it, it will disappear at the end of the current step (or phase).