this is really a wonderful (in hindsight) example of why EVERYTHING needs to be in writing. If a deposit is made, they should be signing an agreement stating what it covers, terms of any refunds or if the deposit is in fact, non refundable. Making them all non refundable if they change their mind makes them think way more about their orders.
for any custom orders I do, I require a fifty percent non refundable deposit. this covers some of the supplies I need to purchase, time for set up/design, blanks that may be ordered and the time my machinery is now "held up" to do their order and cannot do anyone else's, making my schedule back up. All they need to see is fifty percent nonrefundable.. explaining more than that at the time gives them "wiggle room" and possible legal loopholes as they could say "shirts hadn't been ordered, so subtract that, or , no paint/ink/thread was used yet so no supplies... and on and on..
so "50% non-refundable deposit on order, balance on completion" is my standard. simple, easy to understand and clear if there's a legal issue later.
Ask yourself "what would judge judy do?" with your stance?