"Umi no Naka Kara Otayori wo" (Letters from the Sea)
It's definitely a swimsuit set (please bikini michelle.........).
Edit: I looked at the colors on the envelopes. Doesn't look like Michelle is in this one........
Member predictions: Tae obviously; definitely Kanon, given the jellyfish on the left envelope and the blue color, or maybe Hina; probably Maya (green on the middle envelope); Nanami?? (orange); and then that last one is hard to tell what color it is
And the preview for the cover of "Blue and Summer"