Depending on how the item you buy comes packaged, you need to have space in one of two areas. If it's in a package that you have to open to get the included item(s), it will go into your consumables, so you need to have space there. Otherwise, it will go to the Mods section of your inventory, so you need to have a space there. Also, if it does come in a package, once you open it, the included ornament(s) will go to the mods section, where you can then unlock them, which will remove them from your inventory, freeing up those spaces again.