Complaint Review: GreenSky Sun Trust Bank

I have made 23 looan payments to Greensky Sun Trust Bank. I pay all myt creditors via Bank of America 'bill pay'. Everyone except Greensky post the payment same day received. Greensky always takes at least 1 week to post. They have taken as long as 6 weeks. My last payment was received by Greensky August 16th 2018, and has yet to post to my account as of August 27th 2018 ... The issue I have is that I am not getting credit when payment is made and as a result, Greensky is charging me extra interest. This is certainly dishonest business practice and I'm hoping it's also illegal.