Hi seren57,
I don’t know anything about the mattress you purchased or where you purchased it so I really can’t make any comments about the mattress itself, the price you paid, any “discounts” you received, or whether they would have gone any lower but I can tell you that in stores where negotiation plays a role in the price you pay you will generally pay too much. There is more about “negotiating” in post #6 here and the videos that it links to.
Unfortunately though, if you’ve already made your purchase it may be a moot point.
Phoenix