Grass Mail is an item in the “All mail” category (Mail). It costs ₽50.
In-game descriptions
| Diamond / Pearl · Platinum · HeartGold / SoulSilver | Stationery featuring a print of a refreshingly green field. Let a Pokémon hold it for delivery. |
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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