Why do some advance chapters cost coins
A subscription gets you ahead of the public release schedule, but not every advance chapter is included. Some are free with your subscription and some are premium. Premium advance chapters cost coins to unlock, just like premium chapters in the public archive.
Why a chapter is premium
Authors decide which chapters sit behind coins. Charging for a chapter is how many authors fund their writing. Early access changes when you can read a chapter, not what it costs: a premium chapter is premium whether you reach it early or after its public release.
The free and premium mix is shown before you subscribe
Every tier card lists the advance chapters available to you right now and splits them into free and premium. A card that reads 3 advance chapters right now, 2 free and 1 premium means two chapters come with the subscription and one can be unlocked with coins. You see the mix before you pay, not after.
When a tier lists more than is scheduled
Sometimes a tier lists more advance chapters than the author has scheduled at that moment. The card shows how many are available right now, which can be fewer than the number the tier lists.
Unlocking a premium advance chapter
Premium advance chapters use the same coin unlock as any other premium chapter. An unlocked chapter stays unlocked after its public release. Coins are spent on the chapter itself, not on the early access window.
Drafts are not advance chapters
A draft is a chapter the author has not scheduled yet. It has no release date, so it is not for sale and cannot be unlocked. Only scheduled chapters count toward the advance pool.