Understanding data model of Invoice

I'm self-taught, so am not fully educated on the need for Invoice Line Items. 

I need a custom solution with a Purchase Order which has one Vendor and many Services.  Based on what I see with Invoice Line Items, this is what I envision:

1. Purchase Order Entity
2. Vendor Entity
3. PO Service Entity
4. PO Service Line Item

I'm not quite sure why I need #4, but that seems to be the only way to have many services on one purchase order and maintain the relationship with Service.

So, I would have the following:

1. Purchase Order has 1:N relationship with Service Line Item.
2. Service Line Item has a lookup to PO Service.
3. Purchase Order has a Sub-grid to PO Service Line Item. 

Does this make sense?