At the beginning of 2015, I tried selling statewide BirdsEye(TM)-format maps on MicroSD cards. Because Garmin locks your GPS's firmware, you need to install an unlocked copy in order get maps from anywhere other than their paid BirdsEye subscription service. A naked cash grab if there ever was one; Garmin would apparently prefer to milk its captive market while it can, rather than take steps which might delay or prevent smartphones from eating the consumer GPS market for lunch.
At any rate, the locked firmware is enough to prevent someone like me from selling map cards through a major retailer. Sales from the CalTopo store were never huge, and always seemed to come at inconvenient times, like when I was on vacation or similarly indisposed. Because I was selling physical products, an annoying degree of paperwork was required to report marginal amounts of sales tax. I was always worried about losing a thumbdrive in the mail, or sending it to the wrong address due to a typo.
It's past time for me to move away from selling physical products. Instead, all of the JNX files I'd sold on thumbdrives and SD cards are now available as downloads to pro-level subscribers. Simply click on the new downloads tab of your account dialog, and then on an individual layer link
Newer maps are broken up into 1 degree by 1 degree blocks, and named based on the southeast corner. Older maps have custom boundaries as illustrated here and here.
Although this is only available to pro-level subscribers, a 1-year pro subscription costs the same as the SD cards I was selling, but gives you access to all the maps rather than just one state.