Hi dudeitsmattk,
Mattress toppers are most often considered to be a personal item and it’s common that there is no return policy at all. For those that do include a return/exchange policy then the cost of returns and exchanges would be built in to the price of the topper so the people who don’t return a topper would be paying for the ones that do. The topper guidelines include this link to a list of some of the suppliers with good return/exchange options. With toppers that can be returned by courier (which most of them can) the cost of return shipping isn’t significant (usually under $50).
The better options or possibilities I’m aware of in the Los Angeles area are listed in post #2 here but I don’t have a list of which ones sell toppers or their return policies so you would need to check their websites or call them to find out which ones sell a topper that would interest you and the specifics of any return policy they may have.
Phoenix