くださる is the honorific version of くれる
ん is from の, explanatory
じゃ, contracted では, usually in the form of rhetorical じゃない, or the slangy じゃん
My problem here:
Is ない left out of the sentence? I think I've only seen じゃない/じゃん so far.
くださる is 尊敬語 unless I had been hopelessly mistaken for months. However, this くださるんじゃ somehow looks, and sounds really casual to me. Is the speaker sneaking in a hint of 砕けた言い方 in the guise of 敬語? I also feel that by speaking this way, the speaker is hiding a sense of affected ignorance of responsibility, but I may be reading too much into this.