Part of the problem with buying large/bulky/heavy items online is the shipping cost and how to dispose of the old one. I know some cities are very particular about what you can put out in the trash. That is something to consider when making that type of purchase (washer/dryer, mattress, furniture, etc.). If you have no practical way to dispose of the old product, then what? If you purchase the item from a local store who delivers also ask them about removal of the old one. That is what we did when purchasing new mattresses recently and they also cart away the old ones and setup the new one in a second floor bedroom. With something like a mattress, there are also so many styles and level of firmness, you almost need to go to the store and see which one you like best.
Saving a few dollars online isn't the only factor to consider.