Important distinction
Do not build a budget around vague promises. Treat something as part of the plan when the contribution is specific enough to rely on and the person has confirmed what they will actually provide.
Contributions can reduce more than cost.
Someone who owns the drinks table, transports older relatives, prints programs, watches children or coordinates leftovers may remove more pressure than a small cash contribution. Record the work as real help so it does not disappear from the plan.