Firstly you have to create a custom print format for sales_invoice in that you can add your custom fields as per your requirements. And after that, you can fetch the Customer_SKU number on the following item table in sales_invoice.
like using these 2 links,
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refer to this link to create a new custom print format.
https://docs.erpnext.com/docs/user/manual/en/customize-erpnext/print-format