How to add meta to email subject and body?

We’re using Email Template Customizer and Woocommerce Order Tracking. We’re also using Woocommerce PDF Invoices plugin by WP Overnight.

We need to add some meta data from Woocommerce PDF Invoices plugin by WP Overnight. We’ve checked the documentation but couldn’t find the right hook and way to include the meta into body and subject.

Below is an example meta from Woocommerce PDF Invoices plugin by WP Overnight. For ease, I’m putting it in JSON format.

{‘id’: 4932, ‘key’: ‘_wcpdf_packing_slip_number_data’, ‘value’: {‘number’: ‘16947’, ‘formatted_number’: ‘PS016947-DX’, ‘prefix’: ‘PS’, ‘suffix’: ‘-DX’, ‘document_type’: ‘packing-slip’, ‘order_id’: 62391, ‘padding’: ‘6’}}

We need to use the packing slip # PS016947-DX in the email body. Please advise which hook or way should we use?