Customary Payment for Building more than one?

Hello,
Let's say I buy plans and build 2 boats with the intent to sell them
from the start. I sell both boats. What is the customary payment back
to the deigner? (or is payment to the seller of plans). Do I pay for
the plans again on the sale of the second boat or a percentage?

Is there anyone here that actually sells plans and has this answer?

Thanks,
Rich