The satchels are now pretty universal, and are ok to use for express (they used to have two different types, but now it's all down to the label and service purchased) - I'm not sure about eBay labels, but the express ones through My Post have a yellow header if you print in colour (the standard post ones have red).

I always put a bit of express tape on both sides of the satchel, just to make extra sure it doesn't get sorted into regular post by mistake - my PO gives me rolls of the tape as needed, but they should put a bit on there for you at the counter if you ask.