We're looking for a reply envelope that would fit inside a standard #10 but with a long flap. Does the #9 remittance work?
Yes. That is exactly what a #9 Remittance Envelope is. A #9 is considered a "reply envelope" and is often found inside the outer mailed #10 envelope. When looking for extra space with a long flap, the #9 Remittance Envelope is exactly what you need.