A list of all commands can be found on the Command page, here

Allows the usage of /customitems inspect when holding the item [item]

Allows the usage of /customitems inspect when holding any native Minecraft item

Allows the usage of /customitems inspect when holding any item

Allows the usage of /customitems info [item], as well as /customitems inspect when holding this item

Allows the usage of /customitems info [native item], as well as /customitems inspect when holding any native Minecraft item

Allows the usage of /customitems info [item], as well as /customitems inspect when holding any item

Allows the usage of /customitems list

Allows the player to see the category [category] in /customitems list
Does Not grant permission to view the items in the category

Allows the player to see all categories in /customitems list
Does Not grant permission to view the items in the categories

Allows the player to see the item with the id [item] in /customitems list
Does Not grant permission to see the category that it's in

Allows the player to see all items, or, if you're using categories, allows them to see all of the items in the categories they have permission to see

Allows the player to see all items in the category [category]
Does Not grant permission to see the category itself. You should almost always also give the permission customitems.list.category.[category] with this permission

Allows the player to see all items and categories in /customitems list

Allows player to craft recipes one of "permission(s)" set to [recipe]

shaped: false
ingredients: #...
permission: "myRecipe" # The player needs the permission customitems.recipe.myRecipe to craft this item
prefixPermissions: (true/false | default: true) # set this to false to make the permission not be prefixed by "customitems.recipe."
noPermissionMessage: (optional)

Allows the item [item] to be soulbound if and only if soulbound is set to true in the item's .yml file. Give the permission customitems.soulbound.* to make it so all items with soulbound set to true take effect for the player.

REMEMBER: soulbound must be set to true on the item!

Allows an item to be soulbound if and only if it is in the category [category], and soulbound is set to true in the item's .yml file

Allows the usage of /customitems get but NOT /customitems give

Allows the usage of /customitems <give/get> Does not allow the player to run /customitems give @a

Allows the usage of /customitems give @a <item>, which gives all players <item>

Allows the usage of /customitems take

Allows the usage of /customitems repair

Allows usage of /customitems create and /customitems edit

Allows usage of /customitems scan

Allows the usage of /customitems setblock

Allows the usage of /customitems drop

[silent] argument in 4.0.16
/cui drop <item> <world> <x> <y> <z> [amount] [naturally] [silent]
 
For your example, this will drop the item silently
/cui drop coal Main -31 75 -69 1 true true

Allows the usage of /customitems reload