We store signs for some little league complexes and some real estate companies. We also store their posts and sleds. All of it..... No Charge. It's an added incentive to buy their signs from us. We have more than enough room to do this and have been doing for over 25 years. We also store a complete convention's worth of signs and displays in our basement. It takes up about 1/6 of the basement's space. We charge $165 a month for that one. They WERE renting 2 storage units and having to go there, pick it up and later return it to them, it was cheaper for them to rent a portion of our basement.
I would not charge by the post, but by the month. Tell them they can have X amount of space for $35 a month, regardless of what they have there, until it exceeds the space allotted. Remind them, then everything is at your fingertips and no going back & forthness will waste their money.