I suggest you create some test data and scenarios and play
stump the model.
Here are some of the facts --you can add/adjust/remove as needed. Note I have used Products and Items in the facts. I have not included Invoicing since it isn't in your model. I did not include Dates with Order or Deliveries, you can add details where needed.
You have a number of Suppliers
A Supplier may supply 1 or more Products
Each Supplied Product has a PurchasePrice
You have 1 or more Customers that can be uniquely identified.
A Customer creates 1 or more Orders
An Order may contain 1 or more Products
An Ordered Product has a SellingPrice
You may negotiate an AgreedToPrice for a Product with a Customer (sale/clearance/loyalty)
An Order may be "pickedUp"/"carried out" by the Customer
An Order may be Delivered to Customer (may consider a ShipTo address)
Delivery(s) may be for full or partial Order (all or some of the OrdererProducts)
A Delivery is made by a Courier
A Delivery includes a List of DeliveredItems
The Customer is provided the list of DeliveredItems
The Customer verifies the receipt and signs a copy of the List of DeliveredItems
For scenarios you could consider the following, and adjust as necessary to create "business scenarios". You could add more complexity, but suggest you start with this and add detail as the model gets vetted.
Supplier X offers Products P1, P2 and P3 with unit prices $20, $25 and $40 respectively.
Customer CustJim makes an Order for 5 units of P2 and 1 unit of P3 and wants it delivered.
For some reason only 3 units of P2 are available. So FedY -the courier company- makes a partialDelivery of 3 units of P2 and 1 unit of P3 to CustJim.
You can test your model using pencil and paper.
NOTE: I use AgreedToPrice as the Price a unit of Product when sold to this Customer on this Order.
This allows you to change Product prices. It preserves historical Order info related to Prices paid for Product(s). It also allows you to have special prices for certain Customers; loyalty programs; clearance sales and Specials etc...
Good luck.