Hello! I am camping soon and need some extra supplies. I believe that we can make an arrangement that will be mutually beneficial.
That sounds good. I'm headed on a mountain adventure. How about you?
The jungle! To save my dear father, an archeologist, humanitarian, and double-kidney donor, from pirate marauders.
What extra supplies will you need for something like that? 
The water is unclean where we are going, and our throats will be parched, so I need water first. Then food. It's hot in the jungle, so firewood is not of the most urgent need, but still helpful for cooking any varmints we find.
I should be able to help with the water. I'm interested in food most of all.
Great! What is your last priority?
Water is the least of my concerns. I'll have plenty of mountain streams to use.
Okay. Tell me what you think of this. I will get 3 water, 0 food, and 2 firewood. This will give you 3 of your most important item and 1 of your second most important. It will give me one more overall, but I ask for two firewood since that is my lowest-value need.
I can do that.