It's a better idea to create a listing.
Just add to the listing this is for the down payment, order to follow, write the date etc. for what its for.who its for and add the word RESERVE or custom order to it so no one else will purchase in the intrum )
If you invoice her, you won't have an etsy record of the transaction and it could also be considered as fee avoidance been as the order is for etsy..
Good luck