Funny, I've never come across a case of envelopes with the flaps already opened in any size. Must be a special order type item.
Even still, if you want to do 50 or 100 envelopes, great no biggie. But if you're doing a realistic mail job of 300-500, or 10,000-20,000 or more; no one is going to stand there and open the flaps and close the flaps; or just close the flaps to prepare them for the mail machines.
Most printers can run #10 envelopes in small quantities just fine. If the machine can run a #10 windowed envelope without messing it up; then I will start to be impressed.
Also, most other less expensive machines run the envelopes without having to open the flaps. Like the Xante Illumina, although not my favorite machine.