This sucks but it is a hard lesson about dealing with large companies. If any company wants anything that doesn’t comes off the shelf of the store, they have to pay upfront. Pay has to be by a certain amount of days in advance of delivery date or the date is not guaranteed and will be late. Work doesn’t start until payment is done. If they want to pay after delivery, sign a contract, require an advance of at least half of the bill or materials cost (whichever is highest), non-refundable, include a cancellation fee. Put this shit up as terms of service on a website and direct everyone to that page whenever you are contacted by a new client. The larger the client company, the more important it is to be this strict. For you it might be a bankruptcy inducing amount, but to them it will be immaterial pocket change, so you have to hold your ground.