The cost of USPS First Class vs. Priority Mail is actually not that much different. Most of the cost is actually because of the weight of the screens. Priority Mail adds tracking info, which is crucial, and we also get the Priority boxes for free, which would normally be at least $2 each.
We do not use UPS because it is more expensive and they won't ship to PO Boxes, which many customers have. You have to ship a certain amount or pay to have a daily scheduled pick up from UPS. Also, UPS do offer boxes, but they aren't free.
FedEx is not even an option because of how much more expensive they are than USPS.