I agree totally with 4NHOTROD about the courier brokerage fee ripoff. I had many occasions where the brokerage fee exceeded the value of the part that was being shipped. An additional problem with couriers is that they will often hold the shipment at the border until they can track you down to extract payment. They don't get around to calling for days, and if you happen to miss their call the shipment will sit there until you start wondering where it is and call them to track it. I have had shipments delayed at the border for over two weeks because of this.
In the end, USPS is less expensive, you avoid the obscenely high courier brokerage fees, and often you will get your part faster.
But there is a hitch (of course). Some of the vendors refuse to use USPS because they have to go to the post office instead of just having a courier pick it up at their shop. My solution to this has been to simply not use those vendors.
I have noticed an unsurprising correlation to more general attitudes to customer service; those vendors who will make the extra effort to ship USPS knowing it will save their customer a lot of money will also make extra effort in their other dealings with me.