You know she would love it. You do not know her size - and asking ruins the surprise. So do not ask.
Three steps - one from you, one from her, one from us.
Pay as usual and leave a note for her. Nothing ships yet - no address needed, no size guessed.
Your note arrives by text or email with a link. She sees the piece, picks her size and color, adds her own address. She never sees the price.
[Wrapping and a handwritten note included / a small extra] - Jack sets this. The moment it ships, you get a quiet confirmation.
Before it ships, her link also lets her swap the piece for anything else at the same price - or take it as store credit. No returns, no awkward conversation, and you never find out unless she tells you.
After it ships, the usual returns apply.