You have him/her delay payment until s/he is ready for you to ship it. (If you have your Preferences for Items Awaiting Payment set to automatically cancel after x days of non-payment, add the buyer to the exemption list at the bottom of that page if it will be more than the x days you have it set for).
If you have the BIN set to require immediate payment you'd need to revise the listing to remove that to make it work.
