USPS Shipping Assistant, free software
I recommend all traders download this program from the post office. Here is the link to the page:
Here are the reasons:
It has a built in address book and as soon as you get the address of your trading partner you can enter it in the address book by using copy and paste. Then save it and when you are ready to send your end of the trade all you need to enter is the weight of the package and by trying the different options you can see which method will be the cheapest and fastest. This software calculates the postage for you, but there is no option to pay for the postage online. You will have to pay the postage the old fashioned way unless you send everything Priority and use the USPS's Click n'Ship online.
When you are ready to ship your end you can print the label on your computer's printer and it will have a barcode on it which EXPEDITES your package. The software will calculate your costs and delivery confirmation is included at the .14 cent online rate instead of the .45 you pay at the post office.
To make your life even more simple, by being online you can easily schedule a pick-up of your packages from your postal carrier if at least ONE of them is Priority Mail. It is a good idea to stock up on all denominations of stamps by either ordering them online from the USPS or by buying them at the post office. If none of your packages is going by Priority you can still hand them to your carrier or put them in your mailbox to be picked up just like a letter.
Warning: I would not take my packages again into the post office to mail as they will weight them and charge you for them WITHOUT the delivery confirmation or CHARGE you extra for the delivery confirmation. It can cause the postal clerks to get all flustered because they may not be used to this.
This is caused by a glitch in the USPS. They developed the software without an option to remove DC. The USPS representive I talked to about it said to just pay for it as if it didn't have DC. HOWEVER, it is tracked by the PO as if it HAS the delivery confirmation, so I prefer to pay for what I get even if the PO can't understand or require it which is why I recommend putting your own stamps on or just giving a check to your postal carrier with the packages.
NOTE: If you have many packages coming and going it is a good idea to have a good relationship with your mailman/woman. In my case, I live way out in the boonies and do not get into town often so our mail carrier is an important link in life.
Back to the software, there is even an option that lets you know exactly the day and minute your package was delivered to your trader.
One more thing: Every trader should make a modest investment in a postal scale. If you sign up at stamps.com they will send you one as a free gift for trying them out. You can also get a good deal on an inexpensive digital scale at eBay.
Here is a link that might be useful: USPS Shipping Assistant