EDIT: This no longer lets you continue the streak, looks like blizz changed it to just turn off jobs. So instead of fixing the problem, they removed the workaround.
you can use this macro to cancel the delivery quest, and still stay on your streak.
/run iAbandonQuestID = 83055; C_QuestLog.SetSelectedQuest(iAbandonQuestID); C_QuestLog.SetAbandonQuest(); C_QuestLog.AbandonQuest();
That will abandon it and give you a new one right after.
This script was posted Graveblossom in another thread, i just changed the quest id to match the Signed, Sealed quest.