There's no direct dolmus service to Kas. If you use dolmuses you'd have to change 3 or 4 times and it would take several hours.
However if you use the Bati Antalya coastal road bus service you'll be there in around 2 hours.
You catch them from the main Otogar ( yes, near Erasta ). You can book your ticket in advance using their website
http://batiantalyatur.com.tr/ or, in most cases you can just turn-up and go on the next departure. As you're definite on your date I'd pre-book a seat. There's a departure on Sundays at 11.00 and it costs 16TL for a single ticket. It gets into Kas around 13.00.
When using the website, check the schedule for the Sahil / Coast route to Antalya and it'll give you departure and arrival times and ticket prices to Kas..