So you buy a nice two shelf bookcase but it’s wobbly and you know it won’t hold much. I’ve recently gotten one and my solution was to put L-brackets on it. After installing about 8 of these brackets at the cojoining parts of the shelving, it is now completely stable and ready for use.
If you want to be unethical you could buy a mattress from Walmart (maybe Amazon?)shipped to you then return it online. They will tell you to just keep it