Why rent a lift when the box truck roof is right there?

Those delivery guys are doing what plenty of truck drivers do, but that doesn't make it right. Cab roofs on box trucks aren't designed to carry a person's weight. They're thin fiberglass or aluminum skins over empty space, no framing underneath like a house roof. Some hold up, some dent or crack the first time someone puts thier full weight on one, and you've got no way of knowing which yours is untill you're standing on it. Your instinct about it working fine until the one time it doesn't is exactly correct. The harness thing also doesn't really solve anything up there. You'd be tying off to the same flimsy structure you're standing on, and if the roof caves you're coming down with it. A harness only helps when your anchor is genuinely independent and strong. For a 14 foot mounting height on vinyl lettering, you honestly don't need much. A 20 or 24 foot extension ladder gets you there with room to work safely below the top rungs. Sign installers hang lettering off ladders every day. The trick is taking your time with layout. Snap a chalk line or tape on a level guide at the bottom edge first, then work top down so gravity keeps the vinyl from folding back on itself. A second set of hands on the ground holding the ladder and handing you the application tape makes it go way faster. If the job keeps growing, look at ladder jacks and a plank, which gives you a stable platform for maybe a couple hundred bucks, or a rented push-around mast lift if the ground is flat. Those rent cheap compared to a boom and you can be up and down in an hour. The twenty minutes those guys spent was them buying speed with risk. For a paid job, eat the small cost of doing it on a ladder. One slip off a cab roof and your side gig is over permanently, along with alot more.

Gotta push back on one thing in the ladder advice. For flat vinyl lettering you might not need to climb anything at all. I've done a couple storefronts with a long application pole, basicly a squeegee head on a telescoping stick, and it handles lettering way above head height fine. You still tape out your level line and you want somebody on the ground reading measurements to you, but your feet never leave the pavement. Honestly that's the part nobody stresses enugh, working a big panel at the top of an extension ladder solo is where a job like this goes wrong, not the getting up and down part.

I helped a buddy put lettering on his shop window last summer and the scariest part for me wasn't even being up high, it was the smooth concrete under the ladder. Every time I climbed it felt like the thing wanted to walk sideways a little. I ended up having his kid stand with a foot on the bottom rung the whole time. Do those rubber feet on the bottom actually grip smooth sidewalk, or is there some trick to keep the ladder from sliding?