Thanks Mail is an item in the “All mail” category (Mail). It costs ₽50.
In-game descriptions
| Black / White · Black 2 / White 2 | Stationery designed for a thank-you note. Let a Pokémon hold it for delivery. |
|---|---|
| X / Y · Sun / Moon · Ultra Sun / Ultra Moon · Let’s Go, Pikachu! / Let’s Go, Eevee! · Sword / Shield | Stationery designed to make it easy for you to express thanks. Have a Pokémon hold it for delivery. |
Where to find it
Not placed on the atlas maps (so far: Red/Blue and Generation 3).
Sources : PokeAPI (items, item_flavor_text, machines) · atlas · pokeemerald species_info.h