Product_ID in my Invoice table
For what you're trying to accomplish, you'll want to normalize your data into two Invoice related tables. One for header information, e.g. Invoice_Id, Invoice_Date, Customer_Id, TotalCost; and one for the invoice line information, e.g. InvoiceLine_Id, Invoice_Id, Product_id, Quantity, Price.
The InvoiceLine table will have one row per product that was purchased on this invoice. It will have a unique InvoiceLine_Id per row (key field), and the InvoiceId field in it can be repeated across rows because it will be a many-to-one foreign key relationship to the InvoiceHeader table.