Tracking payment is too complex, and might require a warrant. License plate reading cameras are absolutely everywhere now. Even in our neighborhoods. If I were a jack boot, I'd just use these cameras to watch the parking lots of gun stores near the border for CA plates, and flag these plates if they cross back into CA on the same day. Let AI do the job. I wouldn't be surprised if this is already done, though there might be some legal reasons it isn't, like the interstate surveillance by state/local agencies.
Work in 2 man teams. Switch cars offsite.