All you can do is to send the buyer a message and explain the delay. Ask them if this will be OK if not you would need to cancel the order, and then relist it when you're back from holiday.
If they don't mind I would suggest not to add tracking when marking as dispactched, this will avoid a late delivery mark. You'll always have that number to hand, should an item not received be made at a later date
If necessary, hopefully , the buyer will agree to a cancellation and you can cancel the order citing Buyer Requests to cancel this will not give you a damaging defect on your account.
It's always best when going on holiday to end all your listings well before you go away so you can deal with any potential refunds . They will go back into your unsold listing for 90 days if you're on the Seller Hub, and you can then on return, relist them at any time during those 3 months. I prefer to do this rather than use eBay's Time Away option.
@emle_62
