I have had a shipping business for almost 30 years. I rarely send anything of any size or value thru the post office. It is not that they don't deliver, it is that if there is a problem, it can take you 6 months (if at all) to get a resolution. The reason most people find UPS and FedX ground more expensive is that they go to a chain store that increases the actual charge - this is perfectly ok, this is how they stay in business. The old Mail Boxes Etc doubled their shipping cost. You need to make your package as small as possible (it does go dimensional vs. actual weight). Included in cost is $100 insurance, a tracking # and a set delivery day. Either go online and create a label or go to a smaller independent shipping store. If you have an online purchased label, you can drop your box off at ANY place that does UPS - no fee.
Also, you can use (or reuse) any box to ship UPS or Fed x, as long as it has a legible label. I often ship customers packages in priority mail boxes. I also like to put a plastic bag in the bottom of the box to act as a moisture barrier and a note in the top with the to/from address. I can send a 10 lb, 12" cube box from coast to coast with $100 insurance, a tracking number and let the recipient know the guaranteed delivery date for $20. And know that if there is a problem, I can locate the package or submit a claim with resolution within days. UPS and Fed X can not ship to a P.O. Box (post office won't sign for it) but they can send to a RR#. Hope this helps.