First off, the exact card text for Iron Man: Invincible:
When Iron Man attacks, you may pay a bolt to give him +2A until end of turn.
* He gains +3A instead.
This is a "when attacks" ability, which triggers once for each attacking die. It is a "triggered" ability, which would only happen once per trigger.
In this circumstance, for each Iron Man die that attacks, you may pay 1 bolt to increase that specific die's attack.
If it were something like "anytime you may use a Global Ability, you may pay X to do Y", it could be done as often as you had energy to pay for it.