Sometimes the simplest way is best:     I would write to the buyer explaining that

I am  on a trip at the moment, and forgot to amend the listing - say sorry - and would it be okay if I post it when I get back - give date - or would they prefer me to cancel  the transaction and refund?    Some buyers are not in a hurry, some buyers are understanding, only a few will give you a hard time.

Let us know what they say.