When buying quantities of standard fasteners, such as screws or bolts, buy a full box anytime you will need close to a half box or more. The second half of the box will be virtually free. For example, 45 individual screws will usually cost about the same as a box of 100 screws.
Need to replace your trusty old level? You can check it for accuracy. Lay one working edge on a flat surface and check the bubble. Swap it end for end, then check the bubble again. It should be in the same position. Now try the opposite working edge. Take a reading, switch end for end, and take another reading. The bubble should be in the same position. Likewise check for plumb by holding it against a flat vertical surface.