WooCommerce: Custom Variable with shortcode

I'm using two plugins:

• Follow-Up Emails,
• Booster for WooCommerce.

Booster for WooCommerce lets me add a custom field in checkout, then I can add an empty field asking for a customer's ID number (it's for our internal system and all our business customers have one).

The shortcode looks like this:

[wcj_order_meta before="TAX ID: " before="" meta_key="_billing_wcj_checkout_field_1"]

Now, I woud like to add this customer ID number to the Follow-Up Emails plugin so that both the customer and us can see the ID number in the e-mail.

Follow-Up Emails does not allow shortcodes but it does allow custom variables.

Is there a relatively easy way in adding my shortcode to become a custom variable?

Any help on this would be greatly appreciated.

Thanks.

via Chebli Mohamed