meta key >> second level name

Hi,

In the meta data order list I have:

_wc_shipment_tracking_items
» 0
» » tracking_provider colissimo
» » custom_tracking_provider

tracking_provider with the value : colissimo

I’d like to know how I can access the field tracking-provider which depends of _wc_shipment_tracking_items field.

I want the value of the field (=colissimo) to be displayed in the invoice (WP-Overnight plugin).
I just have to add this code in the invoice customizer : {{my_meta_key}}

Thanks