I agree.  We make it so easy to vote in Oregon with mail in ballots and with drop boxes all over the place.  If they haven't voted in elections yet, they probably aren't going to vote if we give them free postage.    
At say 50 cents per envelope, it tells me they estimate that 1.3 ballots were mailed in for 2016, or are going to be mailed in during 2017.  In the 2016 general election alone, 2,051,448 votes were cast and 1,231,843 votes were cast in primary elections (per elections web site showing historic turnout), for a total of 3,283,291 ballots.  So only about 40% of the ballots were mailed in during 2016 (assumptions above).   
For the majority of voters in Oregon, mailing in a ballot is not being done.   I just don't see this being enough incentive for folks to mail in their ballots. 
If they haven't made the effort to vote in the past, given how easy we already make it to vote here in Oregon, they probably aren't interested in taking the time to make an informed decision.