I work at / co-founded a non profit organization that provides a free service pro bono (something like free lessons/consultations, often in groups).
Obviously, since it is free, there are aspects to it that make the quality and organization not as stellar as a for-profit service, e.g. maybe last-minute cancellations or changes, low available staff, etc.
Yet I get complaints all the time: "You should have told us [3 weeks] in advance", "This is very unprofessional", "This should have been done differently [this way] or [that way]", "How is it possible that I have to wait 2 days for the test result?", etc
Often this happens on our social media.
Is there a good (but also diplomatic) way to make it clear to all these "clients" that since it is free we do not have all the resources to meet their quality expectations and/or that they should be grateful that it is free in the first place?