I rather agree with raven9; the image of "empty box" is illogical, it is an oxymoron like loud silence etc. She is sending him "an empty box" or "everything he never gave her" which means she is actually sending him her heart, her longing for him he could "fill up". But in the first place he does not understand or wants her feelings, for him it is " just an empty box" (=a needy woman whose feelings he can not satisfy). But after the dark period, "being too tired to swim anymore" he realizes that he made a mistake rejecting a true feeling, and now he is ready to feel her up with all of his presence. Deep and simple.
I rather agree with raven9; the image of "empty box" is illogical, it is an oxymoron like loud silence etc. She is sending him "an empty box" or "everything he never gave her" which means she is actually sending him her heart, her longing for him he could "fill up". But in the first place he does not understand or wants her feelings, for him it is " just an empty box" (=a needy woman whose feelings he can not satisfy). But after the dark period, "being too tired to swim anymore" he realizes that he made a mistake rejecting a true feeling, and now he is ready to feel her up with all of his presence. Deep and simple.