Hi Astroaurora,
Welcome to the Mattress Forum! 
I found this information after a quick glance at the topper thread here:
SleepEZ allow returns within 30 days, you pay for shipping.
Mattresstopper.com allows for free returns within 30 days.
Sleep on Latex allows for free topper returns within 30 days.
Latex Mattress Factory allows for free returns less a $45 shipping fee.
Sleep Like a Bear offers a 30 day money back guarantee.
Foam Order offers a 90 day return policy, but I’m unsure exactly how strict they are with toppers.
I don’t keep track of such specific details, as these policies are always changing, but at least it’s a bit of a start for you. Most businesses don’t offer a “comfort exchange” on toppers, as the feel of the topper is greatly influenced by the surface upon which it is placed, and a retailer has no control over this.
Phoenix