Flexo print is a very low-cost method for printing on envelopes while they are being made. Combining the envelope manufacture with the printing process saves money by removing the need to overprint the envelopes at a later date. The technology is very like a high quality rubber-stamp, and with recent improvements in technology it has become possible to print full colour graphics and text on envelopes using this process; it's rarely as good as litho overprint, but the quality difference is lost on most outside of the industry. For straightforward requirements such a strapline, return address and postal indica, flexo is more than capable of producing acceptable results at very good prices.
Since this printing method is combined with manufacture, there are some minimum requirements as to the run length. In most instances, a reasonable starting would be be 5-10,000 pieces depending on specification. It can cost as little as a few pennies per thousand if the quantity is large enough to print envelopes using flexo.