@Waqar_Muhammad create the wedding cross sell collection and fill the slots that Zazzle has created for you. No thinking needed.
You need to do the research as to what design style is trending in the wedding space. It's up to you to design the collection the best you can, targeted at a specific wedding theme and promote those products so that they do sell.
There are no "must-have products" each bride-to-be has different requirements - follow the collection's slots, see what other products go well with the theme and promote, promote, promote some more.
Ignore store ranking - it will go down the more none sold products you have in your store but as you get all those sales and self referrals from promoting your products - it will go up (though sales and self referrals are not guaranteed when you do promote so ymmv)
Jump into the wedding space, design something that's outstanding and have fun!
