What I've done in the past is this:
- Determine what sort of deposit you would like to guarantee the custom idea / draft work and if you would need it to be non-refundable.
- Discuss the expectations for custom work, a deposit to get started, and whether or not the deposit would be refundable.
- Set up a listing for the deposit with the terms outlined specifically in the listing.
- After the idea is agreed upon, you could set up a listing for the remaining balance. Again, specify if it'll be refundable or not (my guess is not).
OR
You could set up a custom listing where they pay for everything up front before work starts that is non-refundable. I have done both depending on the complexity of the work involved.