Hi dunkinidaho,
Welcome to the Mattress Forum!
I wish you had found us before making your purchase. The major brands such as Sealy/Stearns & Foster, Simmons, and Serta all tend to use lower quality and less durable materials in their mattresses than most of their smaller competitors that will tend to soften or break down prematurely relative to the price you pay which is why I would generally suggest avoiding all of them completely (along with the major retailers that focus on them as well) regardless of how they may feel in a showroom along with any mattress where you aren’t able to find out the type and quality/durability of the materials inside it (see the guidelines here along with post #3 here and post #12 here and post #404 here). You always want to make sure that you find out the information listed here so you can compare the quality of the materials and components to the durability guidelines here to make sure there are no lower quality materials or weak links in a mattress that would be a cause for concern relative to the durability and useful life of a mattress before making any purchase.
There should be a law tag sewn into the tape edge or seam of the mattress, usually at the head of the item (sometimes on the bottom of the mattress). Serta shows you an example in the warranty card details here. The law tag should tell you the model name of the item and that should match what you were provided on your receipt.
There is some truth to what you say, and a little truth to what the salesperson said. All new mattresses will tend to feel a bit firmer when new, and they will lose some of their “false firmness” as time goes on. You will also become used to them as well. But in your case I would still want to make sure that your mattress is the exact one that you ordered, and if you were delivered an item without a law tag, I would request a replacement right away, as having the law tag is required for any potential warranty coverage.
Phoenix