I think that USPS rates have gone up substantially to compensate for the increase in larger and lighter packages being shipped more frequently. All of the flat rate boxes are well under 1 cu foot in dimension. If you ship a box under a cubic foot in dimension the domestic rates are reasonable. If you exceed that dimension, then they start going up exponentially. If I ship an item that weighs 6lbs from Seattle to Kentucky for example, and the package measures 12x12x12 inches the rate is $19.60. If I ship the same weight to the same place and the package measures 12x12x13 inches, the rate goes to $29.95. The 12x12x13 inch 6 lb package is $13.20 by FedEx.
If I can cram it safely into a 12x12x8 flat rate box it is $13.95, and in that box it can weigh up to 70 lbs domestically (a cheap way to ship your gold bars, silver bullion, or large rolls of lead solder.
The same sorts of ratios occur for international shipping also, except the flat rate box max is 20 lbs.
I think that what the USPS is tell us is that they want to ship small compact items, and if you want to ship something large, especially domestically, use a different carrier. (FedEx, UPS)
International quotes can vary depending on what the seller is doing on their end. Many sellers will not use first class mail as it is not insurable and can not be tracked, and with Paypal rules the way they are, a seller is putting themselves in jeopardy using it. It is really personal preference for the seller. Since it is their risk, they are the ones deciding if the sale is worth the risk of offering inexpensive international shipping or not. A buyer simply has to say... "I did not get it" and the seller is out postage, listing fees and the selling price of the item.