I don't disagree with Randy at all. Make sure your security folks sign off on this too.
That said, to answer your question, Reg DD does contain some exemptions for advertising via outdoor media at 230.8(e) that you'll want to look at. However, you should also keep in mind that UDAP is the hammer that's currently in vogue, so regardless of what Reg DD says or doesn't say, you'll want to be sure your courier car ad would not be misleading or deceptive.
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Jim Bedsole, CRCM, CBA, CFSA, CAFP
My posts - my opinions