I am a genealogy fanatic. I write a lot of letters seeking information and I find that the envelopes I buy at Staples are perfect for the job. I wouldn't buy any other kind.
We use these envelopes at our church as tithe envelopes (offerings). They were too small to run through a copy machine and a bit difficult to run through my home printer but they're the right size.
Almost every home-based business needs white #10 envelopes. These are heavy enough to be durable and fairly inexpensive when purchased in boxes of 500.