in this case the only option you are left with is to provide authority letter for Fedex / UPS ( as mentioned in the letter) to process clearance on your behalf. Provide them with invoice of payment you have made against your order. otherwise customs will process the duty & tax amount at Total value mentioned in the packing list.
you can locate fedex / ups local agents and provide them with package number to know approx value declared over documents in their possession. Use email, everyone is working from home these days.