Thanks everyone for your suggestions. For anyone else not sure how to handle this, received this response to an email to support:
Thanks for writing in. I'd be happy to help you with billing for these additional charges.
Unfortunately, once a custom order is completed and paid for, you can not adjust the price of the order. However, there are a few ways that you can collect additional costs from your customer.
1) Create a new reserved listing for your customer in the amount you’d like to collect. Here’s our guide to reserving a listing:
https://www.etsy.com/help/article/3662) Send them a revised PayPal invoice. They can pay this using their PayPal account or their credit card. Here's how:
https://www.etsy.com/help/article/3673) Refund and cancel the original order, re-list the item with the appropriate cost, and have your customer resubmit their order.
These articles show how to refund and cancel:
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https://www.etsy.com/help/article/2037--
https://www.etsy.com/help/article/4553Please let me know if there is anything else I can help you with.
Hope you find it as definitive as I did.