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I figured out a work-around for this. I consulted Blackbaud staff on a way to do this and someone referred me to the Letter Templates Library and setting up Letters there that mirror my acknowledgments. Then when you need to do a "one-off letter" you can go to the constituent's record and "Write letter from template" under the task menu, then choose whichever letter you need to print one of. This has been a life-saver for us for returned mail with wrong addresses that need to be generated weeks after the letter was originally generated from acknowledgements. I highly recommend working with BB Support to set this up for your needs.
But I agree - BB should implement a one-off acknowledgment as you suggest. The work-around we had to create to accomplish this need was complicated. It shouldn't be that difficult. An intuitive easy solution should be built into the system.