Actress Michelle Keegan backed a treatment priced at £2.50 in a national rollout.
She described it as a 'godsend for holiday hair' in recent media spotlights.
The promotion provides a £10 discount on the product, which has become popular among her fans.
Wales Online reported a £12.50 version of the mask makes damaged hair 'liquifying' after a single wash and received enthusiastic reviews.
The Liverpool Echo labelled it 'one of the best ever', echoing Keegan's endorsement.
The Mirror, Wales Online and Liverpool Echo all published stories on the discount.
The discount has spurred considerable online demand.