Slacker Radio Plus - data usage



Does anyone else have a subscription to Slacker? I pay 3.99/month and am enjoying it but have an issue with data usage that I can't figure out. When I'm in the house I listen over WiFi and when I'm in the car I stream my offline stations that I've downloaded (using WiFi). The problem is that over the last 2 weeks I've used about 180mb of data when playing offline in the car. I only have a 300mb plan so that hurts. The only reason I subscribe is so I can download and play offline without using data but that doesn't seem to be working. I read online about opening the app when in airplane mode and then turning it off again but that didn't help. Any ideas?