The devotional does not even have to happen immediately within the ceremony (and indeed, the marriage ceremony and the Mass are two distinct things, you can have a Catholic Wedding without a Mass), or even in the same place – it can be done, for instance, at a different chapel on the way to/from the wedding that holds a special significance, or if there’s a Marian shrine in the vicinity they can stop there for it.
White flowers, representing purity, are a traditional Marian symbol and hence offering.