Marlin Leasing Delays posting payments so they can assess "late charges" Philadelphia, Pennsylvania

We received an invoice from Marlin Leasing for our March, 2012 lease payment.  There was a "late charge" on the invoice.  We paid our previous invoice in a timely manner, yet were still charged a "late fee". 

We have not, as of today, received any response to our emails asking for a loan history for our account.  When I check our bank records, there is a time delay of several days from the date we mailed our check to the date the payment cleared the bank. 

Nowhere on their statement/invoice does it indicate when the payment was posted.  Considering the exhorbitant interest rate we are being charged, I would recommend using a different finance company iof you are considering new equipment for your business.