My wife runs a medium sized CSA for the farm she is a field manager at. Typically, they don't do 20# of something at one time. She recommends talking to the CSA operators in your area. I know one farm at a NOFA meeting we went to offered a greens only portion. Another farm would give you your week's "credit", and then let you use that credit a at farmer's market type setup that they did. We don't do a garden anymore, due to the amount of sort outs we get free, but one thing that hurts CSAs in my area is the number of people who actually CAN and DO garden. Now if you had an area that had HOA's or something like that that banned gardening, that could be a gold mine for local small farmers.