Look into USPS flat rate boxes. If it fits in the box it ships for a decent flat rate price. Not good for expensive or large items like subs & amps.
Bubble mailers are also a good easy & cheap way to ship smaller cheap stuff like fuse holders, ring terminals, pieces of wire. They can be had in up to 18" sizes.
For subs & amps I always use UPS. Don't trust the stores to package your items. Some of those employees are just flat dumb. Pack it yourself and always pack tight. You do not want the item to be able to move around freely. Make sure there is some form of packing (foam, peanuts, bubbles) on every side of the product. Pool noodles are great for packing material.