How would you sow the boxed wildflower mixes on a hillside with out the seeds being washed away in the rain?
Most use a seed-and-straw method... scatter the seed and cover with a light scattering of straw (not hay, which has seeds). You can also use erosion mats or hydroseed pulp.
Be careful with boxed mixtures-- are the seeds native? Some mixes contain invasives, or a lot of annuals.