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.
To hold either a round or square post plumb inside a post hole while you pack dirt around it, make up a couple of wedges of 2×6 about 30 in. long. Cut to length, then make an angled cut from a corner of one end to the center of the other end. Insert a wedge on each side of the post; they will adjust in the hole for either post size or hole size.