UNDERDEEP shop skill advance points are paid in friend points, not Coin. The Shop panel in the bottom-left of the interface sells advances in skills and skill slots, and it is not a gear shop: when you open it, the only things on offer are upgrades to your skills.
Underdeep shop skill advance points: what the Shop sells
Two things are on sale there: advances to your skills, and additional skill slots. Nothing else - no weapons, no armour, no consumables.
The price is paid in friend points
The currency the Shop charges is friend points, the gift currency that comes from other players, rather than the Coin you use for town avatars. The price of an advance is not published by the game, and players have asked for the friend-point cost of shop purchases to be lowered, which tells you it is high enough to be a real bottleneck in the early game. Where that currency comes from is covered in the friend points guide.
Forget is a reset, not a refund
Each advance can be undone with the Forget option, and you can rearrange your skill list at any time. Forgetting an advance does not appear to give anything back, so treat Forget as a reset rather than a refund.
Two upgrade paths side by side
| Currency | Where it is spent | What it buys |
|---|---|---|
| Orbs (won in fights) | Skill tab | Individual skills and extra skill slots |
| Friend points (gifts from other players) | Shop | Skill advances |
The reason to buy advances is capacity and reach: more skill slots let you run the skills you actually want, and an advance can unlock skills that the Orbs tab alone will not get you to. What to buy with Orbs once you have the slots is in the skills and Orbs guide.