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It is mainly to streamline things on the seller's end. Postage within the US is based on size, weight and destination of the package. Continental US refers to the "lower 48" and postal rates from Maine to California are somewhat consistent. On the other hand, mailing to Hawaii or Alaska can often cost twice as much. So, if I sell on eBay its easier to say ok everyone in the lower 48 pays $8 postage and be done with it. I got burned once selling to someone in Alaska as it cost twice as much to mail an item there and I used a flat rate shipping fee.
It used to be to mail outside the US you could use the small green customs tag for most "normal" packages. Now, you have to use the "long form" customs form and put it in a plastic envelope, etc. All overseas stuff for the most part has to be taken to the post office. You can't just hand it to your mail man or put it in a drop box. From where I live now my main PO is about 8 miles away, has limited hours and access, and is often busy resulting in a 15 - 30 minute wait at the counter. Not worth it to ship a $4 box of Tamiya figures to someone in Yorkshire.