Yeah, there still should be minimum requirement for it.
Hmm, it basically going to be look like merchant mechanic that look like calculator thingy.
First, there should be a window on how many quest you want to complete. If 1 quest need 30, then 20 quest is the limit since 1 stack only has 600.
So once it register on how many quest, there'll be a "deposit all" button that corresponding with it. The system already calculated on how much it need, and "the deposit all' button will limit it accordingly.
For example, 15 quests, then it need 450, "the deposit all" button will limit it to 450. So you just need to click that button and done.
Something like that I guess.