We test all our stocks in our digital presses and if for some reason something goes wrong with the print job (stock gets jammed or another problem) which can happen, we always have more stock in the warehouse to draw upon.
Also, our price for printing always includes any stock we may mess up or need in addition to set the job up on the press.
When we manufacture our envelopes, we shrink or package them as quickly as possible and keep them in a cool, dry environment. We know what glues we've used in the manufacture and tested they won't damage our presses. We know they haven't been warped or damaged in any way when we put them through due to the care we take in handling and storage.
For these reasons, we don't offer printing onto any envelopes other than our own... (but we do have over 400 different envelopes to choose from!)