Congrats to your friends!
I like the idea of the bottles, but I would put other things in there. A little scroll of the invitation, or ceremony, or other "message in a bottle", tied with a cute little bow, some faux greenery stems, maybe a base of pretty aquarium gravel or sand, buttons or beads, tinsel, whatever fits the opening & the theme.
A similar idea could be little solar lamps made from mason jars. I've done this, not expensive. I bought the solar jar lid lights and jars separately. I glue seashells to the out side of some, put pretty rocks in some, put seashells in some. As wedding favors, the outside could be covered with some pretty sheer fabric and a bow, &/or with some objects or decorations inside.
Check the fit before you commit. I glued shells to one jar that doesn't fit the solar top. At a glace it looked like the same size opening, but it's not.
Nothing to do with what you asked, but on the topic of wedding stuff, I saw a case of tiny bubble tubes (that you use to blow bubbles) recently. Like 50 for $15. The kind of thing that could be on the tables at the reception for guests to blow bubbles. I think it would be fun & festive to have people blowing bubbles @ a wedding.
