There is always a tradeoff between economy and appearance. The letter inside would be individually typed and addressed to the recipient by name. If time or quantity prevents such methods, there are ways to make your appeal appear to be personal. And the more personal your appeal, the more effective it will be. The envelope will appear more personal if hand-addressed rather than labeled.
Volunteers can address envelopes for weeks before the mailing goes out.
An envelope that appears as if it has 20,000 identical twins starts out with two strikes against the possibility that the recipient will want to respond to the message in the letter. The letter itself should be printed, not mimeographed or duplicated in some obviously cheap manner. Again, if the quantity is small enough to handle, sign each letter individually. If you can't, have the printer use blue ink for the signature to set it off from the black type. It should re-state the message of the letter, and move the reader directly to the reply piece.