I agree with what the others have said about USPS.com
If you create an online account you can save approximately .15+
per package on flat rate (not sure if there is a discount for NON
flat rate or not as I only use the flat rate if I am mailing packages
that I want a tracking number for.
And I order any and ALL the sizes of FREE flat rate boxes, and the
envelopes. Why not they are free, and I can almost be sure to have
the right size no matter what I am mailing (at least most the time).
There are 2 different styles of medium and large flat rate boxes I get
them all, never know when you will need a "longer" box.
You can even schedule for them to pick up the package if you so
wish (I normally do this). I love the pickup service because there
are times when I just do not feel like going to the post office.
I DO go to the post office to request the "Priority Mail" packing tape
that they have (it is free but you have to ask for it). I wish they would
put that on the website for ordering also.