ok… forget about biztalk for a minute.
lets talk covast…
all this is done through the covast pipeline component, covast envelope or through context properties of the file.
yes, you dont need a recipient address IFF covast can find it elsewhere. that is, in the envelope (if it originates as an XML) or in the context, if you are doing it dynamically.
That way, in your pipeline configuration, leave the recipient URI blank and just set the sender.
Mind you, you do need to have all the parties configured. I suggest using the Covast administration to add parties and party aliases. I personally have a recipient party for each business application. within these, i have different aliases for each trading party. Within that, you can define the particular element delimiter (in your case its not default(*) its ~) but also the document you want ( an 811 in your case )
If you could, send me the scenario that you are trying to do in particular. you seem to be recieving an EDI file, in which you are configured as several recipients (??) are you then routing them along or saving them somewhere? if you could post a birds eye view of the scenario it would be very helpful