Not sure what you mean by civilian. I have been out since 2000.
I use the mobile app on my phone. Before the app I used deposit at home. I haven't mailed anything to USAA in years.
Mobile deposit is available for all members with a credit line (credit card, etc). If you only have checking/savings, there is no 'mobile' option for deposits. I have to take all of my checks to the UPS Store for deposit, but the service is rfree & relatively easy.