It seems to be a standard to ask for 100% up front... Otherwise (as it's happened to me and I've seen from many others on this team) you could get into a little trouble. Especially with something like wood that you can't just undo it and reuse the materials! :P I think people generally understand that when you are making something special for them, paying up front is a requirement.
I have the policies about custom orders in the listings AND my policies, but chances are you are going to have to spell them out via convo/email to whoever you are dealing with... just the way it is :P I think the wording in the coaster listing is good the way you have it now, because you are telling the customer to message you about larger orders. Then, when they ask you about them, you can tell them how much it will be and ask if should you create a custom listing for them.
I would also have a chart somewhere (I do mine in Excel so I can calculate and double check the percent discount I am giving) with the amount of discount you are going to offer based on quantity. Just because it can get a little confusing sometimes, and it helps to tell the customer something like "For X amount of coasters, I can offer you a Y% discount and shipping would be approximately $Z." Just sounds really professional, and that gives them the peace of mind they need if they are paying you for something they haven't seen yet! (:
Love all your stuff, by the way, it's all very beautiful (including that you work so closely with your kids!) and makes me miss living around so much nature! (:
