Now with the version 4.2.4 of the WP Travel plugin, Coupon code email tag is available.

If you have used the coupon code while buying the trips then adding the coupon code email tag in the email template will make your coupon code available in the email you received.

Tag:

'{booking_coupon_code}'

You need to add this tag by going to Admin Panel > WP Travel > Settings > Email

Email Screenshot: