Amazon India — One plus x mobile / the deliver courier refuse to accept credit card payment

Address:

Kolkata, West Bengal, 700001

I order the above mobile on Jan 21st with terms of payment as COD, As per terms & conditions given in the offer I am eligible for 15% Cash back on CITI card on COD terms . But delivery person refuse to accept credit card payment . Now the online price is higher by Rs 2000/- and I could not get 15% cash back also.
This is my Ist transaction with Amazon and I am totally disappointed . I called 3 times to your customer service but no proper response.

Sandeep
Mobile [protected]