The case sizes vary from one service provider to another. However, you have the opportunity to select as low as 250 envelopes per box. Though it is advisable to always order with a buffer stock like you can opt for 500 envelopes in one go. This can help you avoid the last-minute rush or being scanty while there is a need. You can check the site of Chcekomatic to order such envelopes; they give the option to select regular envelopes as well as window envelopes for checks. The process is the same as you do to order business checks online.