PS  - this Flat Rate - not bulk like previously said - make sure you pick the right envelope - Flat Rate/Priority Mail not Express Mail is guaranteed to be received in 2-3 days and actually those I send here in Texas are received the very next morning.  The out of state ones usually get them in 2 days - so it is very comparable to the federal express 2 day service that most want us to use if it is there service number.  I thought that was an important detail to whether or not you consider using this. One thought is that if you didn't add any added cost to overnighting that you might possibly inform them of your error in their reading their instructions and ask if they would split the cost and give them further information on what you would charge for future orders or if they have a number and courier service that they prefer you to use.