Dexcom in Germany sends overpatches free with each delivery, at least in my state. A pack of 10 overpatches comes with every box of 9 sensors.
I use overpatches more in summer, if I'm more active or if I'm trying a new location. I was on holiday in London and I think the hard water made the sensor adhesive come loose sooner so I used overpatches. I'm currently trying a sensor on my inner upper arm and I applied an overpatch on day 1 as extra security.
I've had no problems with itchiness.