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Are you in the UK? Just pop into your local post office.If not, here's a range of dimensions for standard envelopes (believe it or not, envelopes do have lenghths, widths and even heights...as all 3 dimensional objects do!) - http://www.severnprint.co.uk/resources-box/paper-sizes/Whatever you do....DO NOT USE PARCELFORCE!!!!!!! Fedex and DHL operate in UK so you're covered there.Your average C-4 envelope should weigh around 200-800g, depending on what it is made of, and what is inside - http://www.royalmail.com/personal/help-and-support/Tell-me-about-size-and-weight-formatsGood luck!
I used DHL with no problem. They have a fixed price for a big A4 envelope. I think it was about 37 pounds so not cheap but you can send them at most WHSmith shops and other locations. For an A4 envelope it was cheaper than the post office.