Down here at the shore, I can normally expect to see the first hard frost in November. Sometimes it's earlier; other times it's been as late as Thanksgiving.

As long as the ground is workable, you can plant. Just don't forget to water (and not just at the point of planting for your perrenials). All fall plantings need 1x watering if not more (if it's very dry).

I recall planting a 1/2 dozen landscaped sized hollies (that my sister said were harboring 'bees' and ruined her outdoor gatherings. I told her it wasn't the case, but...). Anyway, I planted those hollies in my place on January 1 one day about 3-4 years ago. I chopped thru an 1" of frost and then dug the holes with no difficulty. The next day???? Frigid temps and winter kicked in with a vengance.

The hollies??? Doing nicely.

Protect your investment. Plant, water, and see it through till the ground freezes.

Since lavender can be hard to overwinter, I would guess fall isn't ideal for it. Perhaps with some mulching or protection, you will do OK.
Every fall I get taken with sales and deals. Some plants make it through but I lose enough of them to perhaps not make it worth it. But I fall for it everytime. This year, I picked up some hosta - Pauls Glory and Gaucamole -I think the risk is on my side here and that blue flowering plant that starts with a p. Strawberry Blossom always sells their small stuff for 99 at the end of season. I've gotten some good stuff this way but I would say one out of five sticks around for the next season.