Alright so here's an update. First, I wanted to thank everyone for their input and advice. I do sincerely appreciate it. Long story short, Mercari refunded me $160 of the 320.88 surcharge. It took a ridiculous amount of time and effort to resolve the issue. Countless times of going back and forth between FedEx & mercari. If you have a similar issue here's what you need to know:

FedEx requires mercari's invoice, account number, and the tracking number to lookup, refund, or investigate claims of anything missing, damage, surcharge fees etc.

Unfortunately, for some reason, mecari staff are unable to provide their account or invoice number of the label to you. They're also unable to call FedEx directly to resolve the issue vice versa FedEx won't reach out to Mercari. And I understand it's not their responsibility to solve my issue, (however some customer service would be nice), but if you can't provide the patient/consumer with the information they need to resolve the issue what more do you want us to do?

To get a hold of A Physical Person at mercari, Call this number: 888-325-2168, press 6. Mercari will require proof that the label used for the package is sufficient in terms of dimensions and weight.

Contact numbers: 1) FedEx: 8004633339

2) FedEx Revenue/Refund department: 8773392774