Well I thought so but having just spent 20 minutes trawling through the AP site, there's plenty of mention of registering letters, but the only reference to SOD seems to relate to satchels or parcels, so perhaps SOD isn't available for large letters although I'm sure I saw mention of it being an option on the AP postage calculator when listing the other day. I'm going to the PO tomorrow so I'll ask them while I'm there and get a definitive answer.