Your items look as if they would go through a letterbox and don't need a signature.
There are several possible solutions.
If you are posting the items without Simple Delivery and with no tracking, personally, I would message the buyer to agree the date when you will send them, but mark the items as sent. This is what I would do and not an 'official' Ebay solution.
Or, as advised, you can ask the buyer is they are happy to cancel the sale for now and to purchase again later - you can always put the items to one side for the buyer.
Or, if the items are letterbox size, simply tell the buyer that you can't do that as it will have a negative effect on your account and send the items now.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)