I found this when I accidentally messed up and had full bags during my Champion Daily. If your bags are full you may complete your riding daily against the champions and your marks will be sent to your mailbox, then you may proceed to clear space in your bags and do them once more and you will get your marks as intended. Once you turn in your quest simply go retrieve your marks from your mailbox and the next day you may turn those in without doing the daily.
This works with both Valiant and Champions Marks.
Also, I'm terribly sorry if this has been posted already I could not find anything about it and simply wanted to contribute a little since ive been using mmowned's services without being able to give much back.