Ordering a hero to purchase an item at a store (not AI)

How can I do it with jass? I already have a trigger that gives computer players items and takes some gold from them, but that just ignores the item cooldown.

call IssueNeutralImmediateOrderById(player_to_buy, shop_unit, id_of_unit_or_item_to_buy)

Example:
call IssueNeutralImmediateOrderById(Player(0), tavern_unit, 'Nngs')
This buys a sea witch at the tavern for player 0. Of course a unit of the player has to stand nearby for this to work.

Thanks, I kinda knew you could help me.