Just to throw another option out there, months back another train rider posted about using the paypal shipping backdoor to pay for postage. I've been doing this for months and love it! I have a postage scale that I got off ebay years ago. I weight the envie (I ship in the small manila envies), go to paypal for postage and print the label. The paypal way does require you get delivery confirmation for 19 cents, but otherwise it's the same rate as going to the post office. My envies are usually in the 4.x ounce range. I round up to 5 oz so I don't run in to issues in the event my or their scales are off. The 5 oz envie I shipped out Tuesday cost me $2.09 and I drop it in the big blue box at work. No going out of the way to the post office and standing in line.
I guess overall I'm paying 81 cents more than what ShiPoo is paying to ship approximately the same weight. For me, personally, the convenience of not having to go to the post office and stand in line more than makes up for that. Not to mention the extra gas I burn to take an out of the way trip the PO.
If anyone else wants to give it whirl, here's the link.
https://www.paypal.com/us/cgi-bin/webscr?cmd=_ship-now
If you try this, select First Class Mail as the delivery method. You then have to select the package size. The only two options are Package/Thick envelope or Large Package. I select the Package/Thick envelope and if you ever go over with coupons with this option you have WAY too much time!! Here's the description....Package/Thick envelope - A package is a parcel less than 84" when you add the length + girth. Length is the longest side of a package. Girth is the distance if you wrapped a string around the two smaller sides of a package. A Thick envelope is a large envelope exceeding 3/4" thickness.