What counts as an optional additional cost for the purposes of purple Ezrael?

Ruling: Optional additional costs must both be optional (using "may") and explicitly labeled as "additional costs." Costs required to finalize a card (like Flame Chompers or Vayne's energy cost) are not additional costs, even if they involve choices.

Sequence:

• Check if the cost uses "may" (making it optional)
• Check if the cost is labeled "as an additional cost"
• Both conditions must be met to qualify

Nuances:

• Repeat and Accelerate explicitly classify as optional additional costs according to the Comprehensive Rules
• Flame Chompers' cost is optional but not additional (the unit cannot be finalized without paying some cost)
• Vayne's ability cost is not additional because the ability would not finalize without the 1 energy being paid
• Clockwork Keeper's cost qualifies because it is tacked on to playing him and he would still be finalized and played without paying the additional calm power