Sealy Posturepedic Plus Plush Pillow Top (Innerspring Mattress) - Help

I tried this in the store and it felt like the perfect mattress. It was supportive yet soft and it felt like sleeping on a supportive cloud.

I have the mattress and it's already sinking in within a week (my partner and I have a combined weight of less than 290 lbs, so it's not that). I was assured from my store I was getting a new and never used mattress because they do not resell used mattresses. Still - it was delivered with someone else's delivery tag from August and I have a transcript saying they would allow a return if I noticed a delivery error (incorrect or used mattress) within 14 days. I noticed it day 1 and now the site blocked me while refusing to give me their copy of the transcript and not accepting mine.

I've contacted Sealy and my card company, but I feel so upset and don't know what to do. Is it normal for this mattress to sink in after a week or is this happening because of the damaged/used product? (I ordered a mattress encasement bc I was fearful of something like this happening, so the encasement went on day 1). I'm also pregnant, so my lower back was already killing me, but now my neck is too. Our last mattress got damaged in storage during a move and also sank in the middle (it was a decade old cheap sealy essential) and that was WAY more comfortable than this mattress. Now I feel stupid having us spend four figures on this uncomfortable mattress AND to be duped by the store who sold it to us. The duping already felt bad for the first few days, but it sinking as of yesterday made it worse.

Is it the mattress or the store's quality control?

In the meantime, we have a ton of family in the area and are thinking to ask someone if we can swap this mattress out for a guest room one or if we can swap it out with one of the mattresses from our childhood bedroom's.