Hi Hi Melanie_KE,
I was writing my own reply when you posted this comment so I didn’t reply to it in my last post.
With a local purchase for the majority of people … careful testing using the guidelines in the tutorial rather than just testing for the more subjective “comfort” of a mattress (which often won’t predict how well you will sleep on a mattress or how it will “feel” when you sleep on it at home) and some good guidance from a knowledgeable and experienced retailer or manufacturer will usually result in a mattress choice that is well inside a suitable comfort/support range and will generally be “close enough” so that if any fine tuning is necessary it would be relatively minor and involve different mattress pads, sheets, mattress protectors, or perhaps even a topper if a mattress is too firm (see post #4 here and post #10 here).
Of course if you can’t test a mattress in person or if for any reason you aren’t confident that a mattress you are considering would be a suitable choice then the options you have after a purchase including the the exchange/return policy (and any costs involved) would be a more important part of the “value” of an online purchase just in case (and in spite of the “best judgement” of everyone involved) the choice you make doesn’t turn out as well as you hoped for.
At the end of the day you will either need to rely on your own careful and more “objective” testing or a more detailed conversation with a knowledgeable retailer in combination with a good trial period and exchange/return policy. Anything else would lower your chance of success.
Phoenix