Because it is a way for paying for the mail or letter to get to the other person. That way someone also gets payed to deliver the mail.
It was a habit. Before there were postage stamps, there was stamped papers. You purchased them, wrote your message, folded on the lines and the 'stamp' was printed on the upper right corner. The address was going to be in the center area of the envelope, so that area wasn't available.
It is the way the automatic sorting machines are set up. If the stamp is somewhere else, the envelope gets kicked out and has to be manually sorted. That can slow the letter down.
Custom envelopes can be a good way of promoting one's business. Custom envelope printing is offered by a number of companies including Vistaprint and Denmore Press.
You can obtain custom postage from the US Postal Service each time you visit the post office. You can also have the fees for national and international services through the official website of USPS.
If you mean custom games you go into multiplayer and in the bottom righthand corner you should see the custom game section.
Preferably with secure wrapping and proper postage paid...
You can create custom postage stamps online at the US Postal Service website, these however might be a little more expensive than some of the other options online. Shopping around for the best deal might be wise.
There are a few places that one could order custom corner cabinet furniture. One could shop at hardware store such as Lowes, Home Depot, or Menards to name a few.
You could probably get a carpenter to make you a custom desk. Or you might luck out if you have a furniture manufacturer near by, but generally, stores don't offer custom desks.
For those seeking to purchase a personalized flask that has custom engraving, there are options available. Sites for this endeavor include Red Envelope, Flasks, and Ace Gift Shoppe.
To find cheap corner cabinets, you may want to check out home depot, menards, or lowe's. Another option is to look for a local handyman in your area. Many can build custom or semi custom furniture for a good price.
It could be similar to the same principle of making custom made papers and cards If you can make honest money at it then by all means do it.
USPS accepts all unused U.S postage stamps since 1861, these stamps are still valid and are valued at whatever price they state, however if no value is shown the the value depends on the particular stamp.
There are many places where one can find printing for custom envelopes. One can find printing for custom envelopes at popular on the web sources such as Print Runner and Precision Envelope.