This might come as something of a shock to you youngbloods out there, but back before the introduction of the GCA (Gun Control Act) of 1968, you could actually buy *all sorts* of firearms by mail order (!). Every outdoors magazine had ads from gun dealers all over the US....all you had to do was to fill out a small disclaimer form (provided in the ad) stating that you were of legal age, not a convicted felon, and not a habitual drunk. Sent in along with a USPS Money Order, your new gun would arrive in a couple of weeks, delivered by your friendly Postman.
Some sample offerings, with then-current prices:
P08 toggle-switch 9mm Luger....$49
S&W Official Police .38 revolver....$44
Browning 9mm auto pistol....$125