*should have
It's a back spelling from should've which many forget is a contraction. "could of" also exists for the same reason.
It's things like this which over time create new languages, at least if populations are cut off from each other for a long enough period.
Sometimes mistakes are due to a lack of reading professionally written works.
I don't recall the exact sentence but I read something akin to this years ago: "I was a shored the package would be there."