There is another fic (or possibly the same fic) that I was looking for that had Spike buying Xander an SUV, or some type of car anyway? And I think they take it for a drive into the woods or something, where Xander's present to Spike is to let him claim him? I think either he found a way to let Spike bite him, or he figured out that it was okay for Spike to do it if Xander was okay with it? Something like that. Or, even, this could be two different fics for all I know. I kind of remember them living somewhere where there is a lot of empty warehousey space, maybe? Where Spike hid most of Xander's christmas presents? I don't know.
I've been reading a lot of classic Spander recently and I think I might be mixing up a bunch of fics together, lol.