Is there a standard rate for finance charges?

No, but most people try to charge 1.5 percent per month with comes out to being 18 percent per year... In my years of doing accounting almost no one will ever pay this. They will just pay the bill that is due and no more... Heck when I worked for HUD they used to take the discount as if they paid their bill early and then would  never paid the finance charge for being late in paying their bills...