Hi anders1011,
As long as the materials and components in a mattress are suitable for compression and roll packing and as long as they aren’t compressed for extended periods of time (more than a month or possibly two) then the reduced dimensions allow for shipping with a courier instead of a common carrier which can certainly reduce shipping costs. It can also be easier to maneuver through narrower spaces and doorways than a full size mattress.
The optional online step of the mattress shopping tutorial also includes more information about online purchases vs local purchases and also includes several links to lists that include the better online options I’m aware of as well.
Two of the most important links in the tutorial that I would also make sure you’ve read are post #2 here which has more about the different ways to choose a suitable mattress (either locally or online) that is the best “match” for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) that can help you assess and minimize the risks of making a choice that doesn’t turn out as well as you hoped for and post #13 here which has more about the most important parts of the “value” of a mattress purchase which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability, durability, and value.
Phoenix