y, heading out of town can be a crazy time wrapping up items getting everything ready to go. So part of it could be vacation prep. I would try one more remind

y, heading out of town can be a crazy time wrapping up items getting everything ready to go. So part of it could be vacation prep. I would try one more reminder, include a copy of the invoice or a link where he can pay you and indicate you won't be proceeding till the payment is made and you'd appreciate it if they could take care of it before they leaves or during his travel (if it's making it online). He might be waiting to respond after he's on his way. Tell him to enjoy his vacation and you're looking forward to making progress on the project while he's out of town. If you don't hear back from him I would give it a day or so to see if he takes care of it on vacation. If not I would wait to get started on it next week after he returns and makes payment. I wouldn't work too far ahead without agreed payments being made and I wouldn't bug him on his vacation. Good luck getting your payment.