Each feature is a one-time purchase for a given option. You choose Spotlight or Premium at the point of purchase.
If you took Spotlight and then want even stronger visibility, you can buy a Premium feature for the same ad to push it higher still.
The price of each option is always shown before payment, so you decide with full information. Nothing happens without your confirmation in the App Store or Google Play.
There's no commitment: you're never required to renew or upgrade.