at least for iOS there are no costs associated with using Apple Push Notification Service (APNS) but depending on the way you use it you either need to pay for

at least for iOS there are no costs associated with using Apple Push Notification Service (APNS) but depending on the way you use it you either need to pay for the infrastructure that sends your notifications to Apple or for a service like OneSignal which does that for you. Not sure what the volume of LE is but I am pretty sure it's a smart move to focus on their core "business" (providing certificates) and let other handle expiration notifications.