In the 2016 presidential election 17.7% of voters voted absentee and another 5.9% voted by mail. 23.6% of voters mailed in their ballot.
https://www.eac.gov/documents/2017/10/17/eavs-deep-dive-early-absentee-and-mail-voting-data-statutory-overview
Interesting chart...
State Total Votes Total votes Early, Absentee, or by mail/That %
Arizona
2,722,660
2,042,084
75.0%
Arkansas
1,048,513
627,411
59.8%
California
14,610,494
8,592,587
58.8%
Colorado
2,884,199
2,722,919
94.4%
Florida
9,613,669
6,555,802
68.2%
Georgia
4,147,181
2,426,261
58.5%
Hawaii
437,697
229,562
52.4%
Montana
516,901
333,666
64.6%
Nevada
1,128,492
781,788
69.3%
New Mexico
804,073
541,730
67.4%
North Carolina
4,690,195
3,055,960
65.2%
Oregon
2,051,452
2,051,452
100.0%
Tennessee
2,545,271
1,679,087
66.0%
Texas
8,701,152
5,452,437
62.7%
Utah
1,114,567
772,888
69.3%
Washington
3,363,452
3,287,705
97.7%