You will have to use a main form with a subform. The main form should be based on tblLoadInvoices. You will need combo boxes to populate both the fkLoadID and the fkInvoiceID. You will have to create the load and invoice records before going to this form. The subform will be based on tblloadinvoicedetail. You will have to use combo boxes to supply values for the fkEmployeeID and fkItemID. You can populate the price field by bringing the correct price with the item combo box. Also, I just noticed that you have the load invoice date in the detail table. That is incorrect, it should be in tblLoadInvoices.
The form for entering invoices should be relatively simple. It should be based on tblInvoice and combo boxes should be used to populate the customer and price type fields. I don't see an invoice number field in that table. I assume that you have one, so you should probably add that field to the table. For the load information, you will need another form/subform setup with the main form being based on tblDays with a subform being based on tblLoads. On that subform, you will need a combo box to populate the employee (driver).
Give the forms a try, and let us know if you have any questions or problems.