Hi Loroi,
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).
I’ve also added to the information in my previous reply to you here about the different reasons that could account for any differences you feel between testing the mattress in a showroom vs how well you sleep on it or how it feels when you sleep on it at home.
Phoenix