If you shop on board domestic airlines operating on international routes, you must pay either in US dollars or in the currency of the destination country.
Advocate Manoj K Mishra learnt this lesson while travelling with his family to Bangkok this summer. He and his family members boarded IndiGo flight 6E-41 from New Delhi on June 5. On board, he bought masala tea and snacks. He offered to pay in Indian rupees but the cabin crew refused, saying they accepted only US dollars or Thai baht.
Mishra thought it amounted to a domestic airline showing disrespect to the Indian currency. Once he was back in India, he sent a complaint to IndiGo on July 7 demanding to know why the airline had adopted this unfair practice which amounted to disrespecting the Indian currency.
Full report here Times of India