Is Deflect an additional cost that can be optionally paid, and what happens if you can't or don't pay it when casting a spell like Stupefy targeting a unit with Deflect?

Ruling: Deflect is a mandatory additional cost that must be paid when casting spells or activating abilities that target a unit with Deflect. You cannot legally cast the spell or activate the ability without paying the Deflect cost.

Sequence:

• When casting a spell (like Stupefy) that targets a unit with Deflect, you must declare targets during the casting process
• The Deflect cost becomes part of the total cost required to cast the spell
• You must pay all costs (including Deflect) to legally put the spell on the chain
• If you cannot or choose not to pay the Deflect cost, you cannot cast the spell at all (it doesn't fizzle or partially resolve)

Nuances:

• Deflect works differently for triggered abilities: when a triggered ability would target a unit with Deflect, you may choose not to pay the Deflect cost, in which case the triggered ability is never added to the chain at all
• Deflect cannot be paid with effects like Ezreal that reduce optional additional costs, because Deflect is mandatory, not optional
• Targeting happens during casting, not after, so opponents can only react after targets are declared and costs are paid