Terrible Service

I have been waiting for a refund for payment on a incorrect customs duty, which was incorrect because Michelle, who has since left the company, sent me the wrong invoice, this is since the 8th Jan 2019. I have called Nikki from the international billing, I have even sent an email to her and the intbilling email address provided and nobody seems interested in answer as to when I’ll get back my refund. I have today called once again I have been passed from one department to the next, international billing to accounts, been on the line for more then 10min and the only peopl I have been able to speak to are receptionists. How many ppl or days does it take to get back my money which I overpaid because or there error!!!!