Can Ezreal's "reduce additional costs" pay for Deflect?

No, Ezreal's ability does not reduce the cost of Deflect.

According to RiftJudge FAQ #9237 and #8416, Ezreal, Prodigy only reduces optional additional costs (costs that use both the phrase "as an additional cost" and the word "may").

Deflect is a mandatory additional cost (Rule 809.1.d). Because it is not optional, it does not qualify for Ezreal's discount.