Reliance Mart — I'm complaining about cash I paid

I've bought red banana from smart point - reliance mart - 641004
Offer -
1kg @ 49rs
2kgs @39rs
I bought 2.255kg and I've paid 98rs.
I've asked this 2.255 * 39 = 88rs this is what the actual bill amount should be but the bill is 98rs. They're giving me explanation that only first 2kgs are 39*2
And for the rest 0.255kg - 49*0.255 = 20.
So, I bought 2 kg - 78 rs (39 for 1kg) and for 0.255kg i paid 20rs.