eBay has recently changed the listing form so it is no longer possible to set up shipping (either flat or calculated) specifically for First Class letters or flats (AKA Large envelope). Set up the shipping section in your listing to charge a flat price for "Standard" shipping.
The categories of "standard", "economy", etc. were set up by eBay to differentiate shipping methods based primarily on delivery speed. USPS First Class mail is considered "standard", which is a bit faster than "economy" services.
