If you are unable to make a purchase in Steam you can try enabling Steam overlay to fix this.
- Go to Steam->Settings
- Click the In-Game option
- Check the "Enable Steam Overlay while in-game" option
- Go to Library
- Right-click on Omega Strikers and select Properties
- Under the General tab, make sure "Enable Steam Overlay while in-game" is checked
If this doesn't fix your purchasing issue in Steam please create a support ticket with our team and we can troubleshoot this with you.