Mariners Finance Sneaky practices--adding huge fees on the final payment Germantown Maryland

I financed a "same as cash" loan through Mariner Finance for an HVAC that I purchased last year.  I was told that if I paid off the loan within one year I would not be charged any fees.  I was not sent a payment book, and there is no way to track your balance online.  Therefore, I divided the balance that I owed into 12 even payments, and had my bank send it over to them on a regular schedule.  When I had a balance of $100 left according to my own records, I tracked down their contact information online and called to see if they were planning to send me any verification that the loan had been paid in full, once I had made the final payment.  --It turned out that I would have needed to pay the entire balance two months earlier than I actually did (i.e., they started counting their "year" 2 months earlier than my loan payments had begun.)  Because of this, they charged me 10% interest for the entire year of the loan--an additional $200 in interest added to the cost of the loan!  This should be illegal.  In this day and age, lenders should be forced to allow borrowers to view their loan balances online so that they can avoid predatory lending practices and fees.  I will never use this company again, and strongly recommend that others avoid them as well.