E.E. Cummings often used lower-case letters in his poems—as well as in writing his own name—for the purposes of experimentation. Cummings loved to play around with words in writing his poems, and often resorted to using lower-case letters as a way of conveying meaning. In this particular case, he uses the lower-case i instead of the usual upper-case one for effect, emphasizing the fact that this poem isn't about him but about the woman he is addressing.

Writing the first-person singular pronoun in this way allows the speaker to retreat into the distance so that the full effect that his lover has on him can be seen in its majestic power. Indeed, she has so much power over him that he responds positively to whatever it is she asks him to do, as in the following:

or if your wish be to close me,i and
my life will shut very beautifully,suddenly,
as when the heart of this flower imagines
the snow carefully everywhere descending;

Note the use of the lower-case i here, how it corresponds perfectly with the speaker's passivity in relation to this remarkable woman who has such a hold over him. There's something self-effacing about a little i; it's shy and withdrawn, doesn't draw much attention to itself. And that's why it's entirely appropriate to the speaker's purposes. This poem isn't about him; it's about his beloved and the extraordinary power she exerts upon him.