Can Ezreal Prodigy pay for Deflect costs, since Deflect is optional?

Ruling: Ezreal Prodigy cannot pay for Deflect costs. Deflect is not an optional additional cost because it does not use the specific templating "may" and "as an additional cost" together.

Nuances:

• For triggered abilities (like reflexive triggers from Falling Star), you may choose to decline paying a cost incurred by Deflect, and the triggered ability will not finalize to the chain (it does not fizzle, as Riftbound doesn't have fizzling)
• For spells (like Singularity), you cannot choose to decline paying Deflect - the cost is mandatory if you want the spell to finalize
• "Optional additional cost" is a very specific kind of cost with specific templating requirements