I do this with my local council that I have a contract with.
I added 20% to the price I wanted and on all my invoices to them its says.
If this account is settled within 14 days of the date then please deduct 20% from the total price.
I have never once been paid later than 14 days because the councils always go for a cheaper price.
I get what i want and they think there getting a bargain, if for some reason in the future there late paying, then I will get an extra 20% bonus.
I had inside help on the planning of this one and it works on most commercials you have a contract with.