Major brands (like Nectar or Saatva) understand the risk that goes into purchasing a bed online, which is why they offer generous months-long trial periods and seamless return policies. However, when these major mattress brands sell on Amazon, it's not always clear whether perks like year-long trials and free returns apply (figuring out how to return a mattress to Amazon can be far from simple). That's why we'd recommend buying direct from the brand, if that's an option.
Check the Warranty and Support section of the mattress listing on Amazon or contact the manufacturer directly to learn what their trial and return policies are if you purchase from Amazon (or any third-party seller). For cheap options that are only available via Amazon, you might need to accept that good aftercare is something you'll have to sacrifice for that lower price point.