In most cases, the answer is no. However, if the letter postage service and package type for your selected provider are available in ShipStation, you can select them and print the postage just as you would for a small parcel.
ShipStation's focus is on small parcel shipping, so whether letter postage is available will depend on which postage providers you have connected to ShipStation and if that provider supports letter postage through ShipStation.
Keep in mind that all labels printed from ShipStation are 4 in x 6 in labels, which are not always appropriate for letter-sized envelopes.
To print letter postage using a Stamps.com or other USPS provider account, choose USPS First Class Mail from the Service menu, and Letter from the Package menu.