Next Day Shipment Problem

I don't do a lot of Next Day Shipment, but when I do it needs to be there next day. Last two times for EliteDresses the USPS took 2 days, so I refunded the customer the shipping for the last one. For the one before, I sent a second dress FedEx Early morning because it was needed for a play. Two happy customers, but I just want things to work! USPS is convenient since I use Priority Mail, but I may need to change my next day to FedEx (not as convenient, but I have never had issues with them).