Don't know what to suggest, but I've been in situations where I obviously wasn't going to meet my goal. I'd just calm myself, accept the fact, then figure out how to plan things better for the next time. It's not the end of the world, just a little bump in the road, plus a good opportunity to learn how to do it better. I've lived through plenty of bumps and learning opportunities while creating my homestead farm.
It's never too late to start! I retired to homestead on the slopes of Mauna Loa, an active volcano. I relate snippets of my endeavor on my blog : www.kaufarmer.blogspot.com
There is nothing humane or ethical you can do at this point.
Either accept your ducks are underweight and size, change prices accordingly, and take them; or wait until they reach size and fill your own freezer.
They were too young or not receiving quite the correct diet or something similar. For next year plan better. Start with your ducklings earlier in your year, or change the diet to help them meet weight and size requirements or both.
This perhaps doesn't address the question as asked, but a couple thoughts:
Ducks seem much less inclined to eat during hot weather than chickens, guineas, or turkeys. Eating produces body heat, which is perhaps disincentive enough. Couple that with the ducks' proclivity to lay on insulating fat, and it's no wonder they don't want to eat much.
Even raising them to an older age may not help. We raise Pekins for our local market. The window for getting a cleanly plucked carcass with Pekins is 8-9 weeks, and again at 16-17 weeks. We had one batch that wasn't big enough at 9 weeks (they were rather skittish and didn't eat enough), so we raised them on to 16-17 weeks, just finishing them last week. They were bigger then, but not by a lot. Even though they were considerably older, they still didn't eat enough because of the heat.
I've become convinced that the best times to finish ducks (at least here in zone 6) are April, when it's still cool enough to allow for ready fat deposits, and not again until cooler fall weather (mostly October). This probably doesn't help if you're raising ducks for a fair competition, but there you go.
hau Heather, please fill out your profile so we know the area you live in, we can give better help that way.
Deb, Wes and Su Ba have all given good advice.
Here in Arkansas I have found that there are some things I have to do for ducks to gain weight once our hot and humid weather hits: make sure their pond is full and in the shade, have several items they like to eat plentiful and in several areas, make sure they have really deep shade for the 115 degree heat of the day.
By doing these things we can see some weight gains, if I can encourage slugs to hang out in one of their areas, they still gobble those down. Other things ducks love to eat are snails, something I am going to work on cultivating as a food source next.
Rice is also good to have growing since they will strip the seed heads and the plant does attract some snails at the base.
As Deb mentioned for a close at hand exhibition it is to late to effect any sudden changes that are humane and ethical.