Unfortunately the answer seems to revolve around whether you look Mexican-American (possibly get fucked with) or Euro-American (they want to sell you stuff but they don't need your senator fucking with business.) Yeah I know about the blond dude that cult killed.
I don't really like Matamoros because I don't know it well, plus it seems sticky. Nuevo Laredo is usually where I cross. My first moves are usually: Buy a bottle of water from a drug store*, then find some dude looking to "guide" me (there he is, right on time,) I'll ask him to take me where they sell comic books (because I know exactly where the kiosk is, a little further in by the park.) I walk to this known spot with my guide/keeper-away-of-other-guides, thank him, ask him to please accept this 20 pesos already in my shirt pocket, then I dawdle, looking at old magazines and comics.
Next up is to tip the halcones, who I suppose include the newspaper sellers and shoe-shiners. Have some polite interactions with them and a reasonable tip for a paper and a shine.
Then maybe a stroll on Guerrero (?) to the nearest bus station. Stay tf outta bars or any place with girls outside yelling for me to come in. Find a really nice tour bus that has paid its dues to the cartels, and float in luxury to Monterrey.
*dates back to my first visit to Mexico. Without the bottle, I am a hapless tourist wearing a sign that says "come annoy the shit out of me because of course I need 40 new hammocks." With the bottle (properly swinging inside the plastic bag) I don't know why, but the interactions suddenly change to normal.