Over the past decade, permanent absentee voting, or “Vote-by-Mail” (VBM), has become increasingly popular. More than half the U.S. states offer their citizens the option to vote by mail. Yet until now, we have known relatively little about which voters are most likely to use this option when it is available, and what (if anything) can be done to convert voters effectively to permanent VBM status. In this paper, we address both these issues. We conduct a field experiment with all registered voters in California’s San Joaquin County as our subjects, where a randomly chosen treatment group receives postcards—a low-cost conversion option—offering them the chance to easily obtain permanent VBM status. We supplement this experiment with a survey of a subset of these voters, allowing identification of what types of voters are most likely to convert to VBM. Though the low-cost postcard treatment did significantly increase conversion to permanent VBM status, this effect shows up at disproportionately high rates among groups that already vote at higher rates.