In the interests of increasing my productivity and getting commissions done in a more timely manner, from today onwards, on the day I begin working on it, I will be providing commissioners with a completion deadline for what they have commissioned me for: if I haven't sent a finished draft of the story by the end of that date, the commissioner gets a refund on anything they have already paid me for the commission.

Even if you're happy to let me take my time on your commission, I'll still be enforcing the deadline on myself.