How do I request payment for an charged optional service once reservation paid for?

[Yvette11]

Hello I've read some of the similar topics and Dave&Deb have kindly provided a link to the mechanism to ask for the extra amount via the resolution centre so it doesn't attract the Airbnb fees - I believe this is it: https://airbnb.com/resolutions

My question relates to charging for towel hire.  The reservation has been paid for and as the guest did not request towel hire at the time of booking I did not send a special offer to include the service.  They have not yet arrived - due in 2 days - and have just requested towels. 
When do I make the request for the extra sum - now, on arrival, on departure?

Look forward to hearing from you, thanks Yvette