The services which offer forever free plan usually make money by selling the user data to advertisers, we don't believe in that. We believe in user privacy and that is why we don't offer a forever free plan. However our plans are still very competitive to market.
PushPrime works on mobile websites, we are in process of introducing suppor for native apps. But currently PushPrime is purely for web apps. PushPrime supports Google Chrome and Firefox on Android OS. No browser on iOS is supported due to restrictions from Apple.
Our representatives will contact you for account upgradation, opt-in will still work on your website but you won't be able to send notifications to users beyond you account limit. Once you upgrade your account you'll be able to send notifications to all or the limit allowed by your new plan.