。
She was kind enough to reply to my letter.
Literal
She [topic-は] courteously also my letter [to-に] reply [object-を] gave.
丁重にも ('courteously / graciously — and') uses the adverb 丁重に with the particle も adding a nuance of 'even' or 'generously.' くれた marks the action as something she did for the speaker's benefit — the benefactive verb 'gave (to me).' The combination expresses gratitude: she didn't have to respond, but she graciously did.