Product: Student loan

I refinanced my home to pay off all my loans, including my loans through Navient ( Sallie Mae ). I contacted Navient after receiving the checks that I needed to send them from my closing company on which of the 5 locations that they have listed would be the correct ones to send the checks two. I had 5 loans through Navient, 3 personal and 2 listed as " Navient Federal Consolidation Loans ''. I was given 2 address one for the 3 personal loans and one for the 2 " Federal '' loans. 

The personal loans were paid and are now showing XXXX balance. There was no activity on the " Federal '' loans even though I had tracking that the check had been received ( Certified Mail ) on XX/XX/2018 at XXXX and deposited by Navient at XX/XX/2018 XXXX. When I called in this morning, I was told by the first person I talked to that the check had been received and was sent to the right department but she couldn't tell me why it wasn't posted to my account. I was transferred to a Customer service rep who then explained to me that I actually had sent the check to the wrong location. This was the location that I had verified as being correct before I mailed the checks out. 

Now I am waiting for the XXXX of XXXX that cashed the check to refund the money that can take 3 to 6 weeks while Navient is able to collect more interest on these loans. 

So quick summary, contacted Navient in late XXXX ( around XXXX ) and got addresses to mail checks. 
Mailed checks via USPS certified mail on XX/XX/XXXX Checks received at locations XX/XX/2018 Checks deposited/cashed on XX/XX/2018 and XX/XX/2018 Deposits showing against personal loans on XX/XX/2018 paid off remaining balance XX/XX/2018. 
Calls from Navient on XXXX asking for payment on " Federal '' loans I report that I have sent them a check and they received it on XX/XX/2018 the rep says they " will note my account '' Contact Navient on XX/XX/2018 and try to find out why the payment hasn't posted first rep says that they are unsure as the check shows as received and deposited. The rep says that is was sent to the right address. Transfers me to another customer service rep. 
New rep looks into account and says actually this is the wrong address and that the loan check should have been sent to a different one. They will refund the money and it will take 2-3 weeks to get the refund. I ask if there's any way to speed up the process as it is costing me money because they didn't put the payment on my account. They transfer me to a rep at the XXXX XXXX XXXX XXXX ( XXXX ) division. 
XXXX rep tells me that it was sent to the wrong address and that I will have to wait for the return check and I'm stuck until then. I ask if there's a way to forgive the interest that will accrue over the next 2-3 weeks since I have sent in a check to pay off the loan. XXXX rep says that " sir it was your mistake that you sent it to the wrong address and the first rep was wrong it will take 3-6 weeks for the refund ''. This was the address that I was told to send it to before I sent the check, I verified it as one of the addresses listed on their site for Federal loans and even the first rep told me it was the right location.

Now I'm stuck eating $ 200+ in extra interest on a loan that should have been paid off last week. This company has the worst customer service and does everything they can to keep you paying for things that are their problems.