Expert 20
Import Excel
Import Multiline Cells
Excel Automation
Excel Automation 2

Alex pointed you to some good lessons. The basic approach is to import each Excel worksheet into its own temporary staging table first. Don't import directly into your production tables until you have verified the data and relationships.

For example, import the Master Parts sheet into something like ImportMasterPartT, the Customers sheet into ImportCustomerT, and the Stores sheet into ImportCustomerStoreT. Then use append queries to move the cleaned data into MasterPartT, CustomerT, and CustomerStoreT in the proper order.

Import the parent records first, such as MasterPartT and CustomerT, so Access can assign their AutoNumber primary keys. Then append the related records to CustomerStoreT, using queries to look up the correct MasterPartID and CustomerID from the values imported from Excel.

The important part is that Excel should contain some stable matching values, such as your part number, the customer's part number, and a customer or store code. Don't try to import AutoNumber IDs from Excel unless those IDs are already part of an established system.

Once you get it working manually, you can save the import specifications and append queries, then automate the whole process with a button on a form.