That may be tough...
That is, simply stating "late fee" may be difficult to enforce if you land in court.
As I mention, GA law allows liquidated damages (late fees)....you can see that code herehttp://law.justia.com/codes/georgia/2010/title-13/chapter-6/13-6-7/
So you can have a contract that provides for late fees.
But to enforce you need to show three things
(i) A determination that actual injury or damages otherwise for the party seeking the penalty are difficult or impossible to estimate with accuracy;
(ii) The parties to the contract mutually intended to provide for liquidated damages; and
(iii) Assuming the parties intended to include an express liquidated damages provision in contract, the amount of the specified liquidated damages must be a reasonable pre‑estimate of a probable loss in the event of a breach.
(See Physician Specialists in Anesthesia, P.C. v. MacNeill, 246 Ga. App. 398, 401 (2000); Caincare, Inc. v. Ellison, 272 Ga. App. 190 (Ga. App., 2005))
For your circumstances #2 is the key. You have to show agreement. That they agreed to the fee, including the amount.
Now...when you set your fee, be mindful of #3...you have to be "reasonable"...it can not be excessive.
What is excessive? THe law is not clear on that...it depends on the case. But over a few percent per day of the invoice would likely be excessive unless you can show a basis.
Example, say the invoice is $100. And you want to charge $10/day late fee...
Unless there is evidence to show that you would incur that damage (say, for example, it is a product whose price fluctuates and you stand to loose if you can not turn that capital
around in a timely manner), that may be tough to justify....since a 5 day late fee could turn into 50% of the invoice.
BotXXXXX XXXXXne: You can charge a late fee...but you need to get them to agree to it including the amount and the amount can not be unreasonable.