We were using another company when I decided to try Staples. I use them for all other business needs. They designed and made 1000 cards for us in 24 hours-unbelievable! Prompt professional service!
I've used many envelopes before but these are absolutely the greatest! They hold stuff and if you seal them the stuff doesn't fall out en route! They're white but you can't actually see through them because of the awesome security features - but cleverly there's a window in the front that you *can* see an address through (pro tip: make sure the recipient address lines up with the window before you seal the envelope). The people I send stuff to also love them (especially if it's a check inside!). Buy with confidence!