Since you are holding Magnus you should simply use it and buy from here
Redeem Titan Gift Voucher - Exclusive for Axis Bank Customers. Earn up to 5% GyFTR Coins & get Extra Discount with Titan E-Gift Cards. Easy Checkout with real time delivery via E-Mail & SMS.
www.gyftr.com
It will count towards yours monthly milestone. If you are not in a rush buy 1l voucher every month. The problem with SBI Cashback card is that after the devaluation it has become a hit or miss kind of a thing. You can buy a small amount on Amazon and check if it gives CB and then make a bigger purchase.