I assume you're speaking of Echinacea plants and not seeds - they are hardy perennials, but if you've just received them, I'd be inclined to harden them off a bit before planting outside, as its got much colder recently. Pot them up if they're not properly planted at the moment, leave outside during the day, ,move into a cold frame or cold greenhouse overnight, or bring indoors to a cool room. Do that for a week, then leave them out overnight, preferably on a night when it isn't freezing cold. Then you can plant them if you want. As for the lilac, again it may need hardening off a bit, so I'd be inclined to pot that too and treat it similarly, though it is pretty tough, so you could chance planting it straight out immediately, or heel it in till the weather's better.