Faulty cot mattress

I called and emailed and still get same reply..fm same personel, exactly what the manager mentioned in the email reply. Luckily for us, we kept the plastic cover and the yellow sticker on the mattress upon delivery.

After more than 2mth from the date of purchase, the seller was kind enough..after much negotiation, and agreed to arrange the supplier to exchange for us.

Bb TK hardly slept in the cot as he seemed to prefer sleeping and resting in the rocker instead.
We only realise it was faulty when Little Aunt pointed out to us during bb 1st month.

Initially, it was to have the exchange at the store, ie we bring the faulty mattress to the store and together with the plastic cover, and the yellow sticker, it is subject to conditions of the mattress for exchange. Eventually, of course, the exchange is at our place, and exchange is only possible, provided we meet the criteria: (1) the yellow label (2) the plastic cover.

Darling spent some time late at night finding high and low for it inside the household shelter after re-arranging the whole storeroom full of Bb's necessities and gifts. I vaguely remembered folding it & placing it inside a bag containing plastic & paper bags.

In the end, after the replacement, the delivery young man didn't even take a look at the plastic cover (or checked if it's the right one), and asked me upon stepping into the nursery, 'what's wrong with the mattress?'

'It's obvious that the mattress is not level by looking with naked eyes.'
The delivery man then went on to remove the mattress from the cot, and I helped to cut open the plastic cover, and he put it onto the cot for me to check the condition of the mattress, as instructed by the seller.

'By the look of it, this new mattress is much level and looks okay.'
He went on to place the faulty mattress into the plastic cover, and despite me reminding him that there's the plastic cover (which Darling had found) at the side of the cot, he used the one he brought istead.

After emailed the matter to the seller, I received a call which I was repeatedly told the same thing: 'yes, we bring the new mattress for exchange, we'll keep one, and you keep the other plastic cover.'
????
Bb TK's cries broke the chain of repeated argumentative phone conversation. Of course, my priority is TK and I politely informed that my baby needs my attention, and ended the call.