Hi card,
A suitable support system under a mattress (either a foundation on a bedframe or a platform bed) should provide a similar level of support as the floor so the floor is a good way to test whether a poor support system is affecting your sleeping experience on a mattress.
Having said that it’s generally a good idea to have some ventilation and airflow under any mattress because it can reduce the risk of moisture buildup and the mold, mildew, and dust mites that can go with it. Having a sleeping surface close to the floor can also increase exposure to moisture, insects, dust particles or other allergens (particularly if it’s on a carpeted surface). It would be OK in the short term as long as you check the bottom of the mattress on a regular basis and give the bottom of the mattress a chance to air out from time to time (say once or twice a week) either by flipping the mattress over for the day or by standing it on its side (without any sheets or bedding on the mattress) so that any moisture or condensation that is trapped in the mattress has a chance to evaporate but I wouldn’t suggest it for the longer term.
Having a mattress on the floor may also be too low for many people which can make it more difficult to get in and out of bed but for those who prefer a lower sleeping surface then I would probably use something like the bed rug here or a thin slat system such as here or the Lonset bed base at Ikea under the mattress to add some airflow and reduce the risk.
The average costs involved with a return/exchange policy are a hidden cost that is factored into the cost of a mattress. It’s similar to shipping costs where a company offers free shipping which is also a hidden cost that is included in the price of a mattress rather than itemizing it as a separate cost. Every online company knows that a reasonable return rate is just part of the cost of doing business online. Many companies donate their returns to a charity and some have their customers donate their mattress to a local charity and provide a refund when they receive the receipt.
While nothing has a 100% success rate … with a local purchase and for the majority of people … careful testing using the testing guidelines and suggestions in step 4 of 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) along with some good guidance from a knowledgeable and experienced retailer or manufacturer that has your best interests in mind 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).
Assuming that the materials in a mattress you are considering are durable enough for your body type and meet the durability guidelines here relative to your weight/BMI range … the choice between different types and combinations of materials and components or different types of mattresses are more of a preference and a budget choice than a “better/worse” choice (see this article). Different people just prefer different types of materials or mattresses but the best way to know which types of materials or mattresses or firmness levels you tend to prefer in very general terms will be based on your own local testing and your own personal experience.
Each mattress category can include hundreds of different mattresses with a very wide range of different designs, different “feels”, different characteristics, and different firmness levels. Individual layers and components in a mattress (including the cover and any quilting material) can vary widely with different thicknesses and different firmnesses and every difference will affect the feel and response of every other layer and component both above and below it and the mattress “as a whole” so each mattress category will generally include some mattresses that have an overall design that will be a good “match” for you in terms of “comfort”, firmness, and PPP (Posture and alignment, Pressure relief, and your own Personal preferences) and others that use the same type of materials and components and are in the same category and may be just as durable but have a different design or firmness level that may be completely unsuitable for you to sleep on … even if it uses the same general type of materials and components.
In other words you may tend to prefer the general feel and properties of some types of materials or types of mattresses over others but it’s always important to do some careful testing on any specific mattress you are considering to confirm that it’s likely to be a good match for your body type and sleeping style in terms of comfort, firmness, and PPP and/or to make sure that you are comfortable with the options you have available to return, exchange, or “fine tune” the mattress and any costs involved if you can’t test a mattress in person or if you aren’t confident that a mattress is a suitable choice.
Phoenix