check out Dan Antonelli....he offers business cards, T-shirts, swag, van wraps, you name it. I bought a 'design' book of his 20+ years ago, but there's now lots of content online.
He does the design and the profitable stuff, the rest he orders for the client, and makes a little markup.
Sure, these days people order their own stuff online, but some might see the value of their time, and avoiding the hassle....especially if they aren't familiar with card stock, poly-cotton blends, and scrim blockout banners!
you're the pro, and they don't have to learn every detail, so will defer to your expert oversight.