Sub Items Creation

Dear Team,

we need as advice regarding one client who is accounting office that provide governmental services for his customers. all items that he sells are services (e.g. create a new TAXID & Tax Profile at Tax Authority). all of his service items have sub items (he calls them procedures) and those sub items has the price and would show in the invoice as child to parent item.

I tried to use Item Group as main Item (Main Service) and Items its as sub item (procedures), however, it does not work hence item group does not have the same behavior as items in sales order and other documents.

also, I tried to find a way create parent and child relational items, however, it does not work as well.

Does anyone has an idea to handle Items and sub items in all doctypes?

Would variants work for him?

I am afraid it won’t, Variant is an attribute of the same item like color … etc.
I believe the only solution is parent/child item.