Research on the effects of prepaid postage on voter participation in Switzerland found a modest increase in turnout between 1.1 and 1.3 percentage points.
Prepaid Postage Using Pre-stamped Envelopes to Affect Turnout Costs
King County Pilots Prepaid Postage in Special Election
During 2017 special elections in the City of Maple Valley and Shoreline School District, King County Elections (Washington state) piloted prepaid postage. While the county estimated 30% turnout based on participation in previous elections, turnout was significantly higher, at 37% and 40%.