Sleepwell — Provided oversized mattresses

I have purchased a twin spinetech air mattress for our king size bed on 22 Aug 2020, At the time of order, a staff from sunrise foam came to our house to take measurement of bed. But at the time of delivery, when I have put the mattress into the bed, it is hardly to fit. Even when I have put it with mattress cover, it is rearly to fit. So, at the same time I have informed this problem to outlet. But they didn't ready to accept his fault, Sometimes they suggest us to use it without mattress cover & bed sheet, sometimes they told us about warranty term & condition about margin of size. And finally they suggest us to use this mattress for 1year after that I have exchanged it to show defect.
In this past 3 months I have visited more than 15 times to his outlet but they refuse to exchange it. I have a simple question
1. If their is condition of margin of 0.5 inch, why your staff has not awared me at the time of measurement.
2. I have purchased twin mattress for our king size bed, so if margin of 0.5 inch in each mattress, them total oversized has 1 inch, which is hardly fit in our bed,
3. Is it right to use the mattress without mattress cover .
I have very excited to purchase the mattress from renowned brand, but it is very disappointing experience for us.
So, I have sincerely request to consider our ecxhange request.

Thanks
Abhishek
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