One item we found when going through Kathy's mom's stuff was this Postal Saving Plan booklet for the purchase of Defense Savings Bonds. This is the low end of the Battleship Booklet series. Completely filling in the fifty 10-cent stamps allowed you to trade up to a $5 stamp. Completely filling in a $5-stamp Battleship Booklet with 15 stamps got you a $75 Defense Savings Bond that would be worth $100 in ten years. There were also Battleship Booklets with $.25, $.50, and $1 stamps to buy savings bonds wroth $25, $50, and $100 in ten years.
The US Postal Service had a savings plan from 1910 until 1966 and the war time savings bonds purchase plan was in effect during WWII. Interestingly, allowing the USPS to operate as a banking system is an idea that could help the USPS stay solvent and help "unbanked" people.