You're right flow it's 'What's It To Be' seventh installment of the Bittersweet series by Herself.
Summary: An ongoing series of Spike/Buffy tales, meant to be read in order. The first springs from «Wrecked» in season six; the series veers off from canon at that point. These stories are het NC-17, with some slashy flashbacks. Winner of Best Series in the 2002 Watching You awards. Nominated in numerous categories in the Fall, 2003 Eternal Devotion Awards. Winner Best Conventional Pairing Series, Outstanding Conventional Author, Round 8 of the Shades of Grey Awards, November 2003.
Beware for those who don't know Herself's work, while incredibly beautiful, it's a lesson in Angst. The last story of this series left me, well... a bittersweet taste so to speak.
That series is on
AO3,
Sunnydale After Dark and
Elysian Fields.