First you have to decide what is important to you:
1) FAST SERVICE
2) LOW SHIPPING COST
3) INSURANCE and TRACKING COST (buyer's responsibility)
4) MOST CONVENIENT
As the story goes "pick any two".
If your item is light weight (under a few pounds) and the cost is less than $50 the best way to ship is probably the US postal system. If you want insurance, you or someone will have to be at home to sign for it or you will need to visit your post office to pick up your package.
What are the odds of a problem?
Our experience is that USPS problems are well under 1%. The insurance costs can be found on the USPS web site. We have copied some data to our web site (click here). The minimum insurance charge is $1.30 for 0 to $50 insurance. Paying $1.30 to insure a $10 item seems like a waste to me. If we assume that problems are as great as 1% and we just wanted to break even on the average, we would never buy insurance because of the postal insurance rate structure. If you don't want to gamble or can't cover the loss it is best to buy the insurance. (One person provided us with two different addresses and we picked the wrong one. He had moved half way across the country from one apartment complex to another one. The post office got the package to him with about a 2 week delay. We are more careful now. The record is 3 different addresses supplied with one order.)
All items are checked for proper operation before shipment. The only way to cover shipping damage is with insurance (buyer's expense). If the item has a hidden problem we will be very embarrassed and replace it for you in an expeditious manner at our expense. We check very carefully and haven't been embarrassed yet but the odds are that a well hidden problem will eventually slip by us.
How about fast service?
We ship the day after payment is received if by US mail. Shipping by UPS may be delayed by a few days because of the distance to the UPS office. UPS ground may take several weeks across the US. Priority mail just takes a few days typically. (We shipped two identical packages at the same time to two friends in the same city in California. One package took 3 days and one took 5 weeks. The post office couldn't explain the problem.) Shipments outside the US are hard to judge. The biggest delays come from customs inspectors. Canada seems to be as bad as it gets. I have seen air mail orders delayed by a month. Israel runs a close second.
Which is the lowest cost?
That depends on your priorities. For small uninsured packages the post office nearly always wins. For heavier packages with tracking and insurance UPS nearly always wins. (For UPS, tracking and insurance are nearly free.)
What is the most convenient method?
This depends on your circumstances. If there is a person to receive the order there isn't much difference. If you receive packages in an area where they may be stolen you will want insurance so that a person has to sign for the package. (We are located on a road with a double numbering system and UPS delivered our two new computer systems to a location 4 miles away. They were left without anyone signing for them. This caused a few hours delay but no other problem.)
We will ship by either the postal system or UPS and will consider other methods for special circumstances. We tend to try for the lowest shipping charges if no other choices are made.