Perhaps a thread on this topic exists? If not, I thought a discussion on which shipping methods are most cost-effective would be in order. If this has been discussed in detail elsewhere, perhaps this should be deleted. If it belongs in a different forum, it should be moved. I'll limit this to USPS, but of course there are other ways of shipping. I'll also limit it to regular 2-3 day (or so) shipping. Of course, there are more expensive and quicker ways.
At the present, in my opinion USPS emphasizes the Flat Rate system to the exclusion of others. I've shipped a lot of boxes, and never sent anything by Flat Rate. I think once or twice it would have saved maybe a dollar. The First Class Parcel option, on the other hand, is a very inexpensive way to ship fairly lightweight items. Yet many people are unaware of it, as I was until recently.
Here is how I would characterize the various options and their advantages and disadvantages. Hopefully others can fix my mistakes.
Priority Mail by weight: 2-3 days, 1 day for close destinations
Very large boxes can be used (perhaps someone can provide the dimension limits). Price goes up steadily with weight. From distant parts of the country it starts at $7.15 for a pound or less; a dollar or more less for closer shipping. For shipping between nearby states, price goes up slowly with weight. For example, 5 lb from CA to NV can be $10 or so, yet over $20 from CA to NY. If one can pay attention to the size, distance and weight, this tends to be a great way to ship. The postage price calculator comes in handy:
Advantages: Large and not terribly heavy items can be shipped reasonably. Shipping is cheap in state or to nearby states. Boxes in many sizes are free from USPS.com
Disadvantages. Weight is so important that one can spend a lot of effort trying to minimize each pound increase. Boxes are not available at Post Offices (at least around here).
Flat Rate: 2-3 days, 1 day for close destinations
One pays to ship a box of a certain size. Weight does not matter, up to a limit of 70 lb., I think. This system is ideal for small and very heavy items. The smallest box starts at $5.95, anywhere in the country. A "large" box is $17.90. I put large in quotes because USPS makes boxes for the regular Priority Mail system that are considerably larger.
Advantages: Boxes are easily obtainable at Post Offices. One does not have to worry (within reason) about the weight of the box contents. Price is predictable--no fighting to eliminate an ounce.
Disadvantages: The boxes tend to be very small, and the price of shipping goes up quickly with the size of the box. A "large" box is more expensive than shipping a larger 3 lb. package by the regular Priorty Mail system.
First Class Parcel::
This is the same rate as sending a fat envelope. Prices start around $2, with a limit of about $4 as the weight limit is approached. The upper limit is 13 oz, including of course, weight of the box. I'm not sure how or if package dimensions differ from Priority Mail. Delivery times, I think, tend to be about the same as a First Class Envelope.
Advantages: The cheapest option for lightweight packages. For nearby destinations, it may be just as fast as Priority Mail.
Disadvantages: It may be slower than Priority Mail, especially to distant locations. Packages are possibly more exposed to heat and cold in transit than Priority Mail. USPS does not provide boxes in any form. Priority mail boxes cannot be used--the sender needs to provide his or her own. If the package goes over 13 ounces, it gets bounced up to Priority Mail.
If people think this is useful I can edit the above as people point out mistakes. Be brutal, but also remember this needs to be as brief as possible. It should be shorter, not longer.