20 x 20 Shed
We see 3 different types of contracts for things like this: (a) a "job" price - in this case the $4000 - which IMO favours the buyer because it is rigid and does not account for more materials and/or time than originally expected (which comes out of your pocket). OR (b) a labour+materials variable contract - where you are essentially paid by the day, he pays for the material. This favours the contractor because he can spend the time he wants at it. Most buyers wonder if this is efficient use of time...
(c) the most favoured type we see is where you quote all your fixed and variable costs, i.e your minimum charges to cover your time, profit, expenses and overhead, mark up the materials by 15%, then charge a variable daily rate thereafter. Everyone wins here. Your costs are covered, you know you won't be losing money and the buyer gets what pays for. No more no less.
On something like this, if this job took you three days, two men, and your minimum "confortable" daily rate with a truck was $800 , then you'd charge $2400+materials. Daily rate thereafter.
Most people go for this. The contractor gets his living paid for working and the buyer only gets what he pays for. Takes the variability of material costs out of the equation.