I Thrive in the Apocalypse by Stockpiling

Evan, heir to an old-money fortune, hides as a warehouse keeper. On the outbreak's first night his girlfriend hands his best friend a rusty axe; they take his arm and throw him to the horde. He wakes seven days earlier: black card thawed, fortress to build, supplies to hoard. When it returns, the two who betrayed him beg while he dines on steak.