Going to use a seatpost shim on my shopper. It’s 100 mm long ,that’s to the top of the tape.
Will I need another shim further down or is the frame suitably reinforced?
It'll be fine.
With that little seat mast, I'd say no. If it was down near your thumb, that'd be much better. Maybe check with the shim manufacturer, but I'd suspect they are designed with the kind of top tube drop you get on a road bike in mind.
Edit - whoa, where did the tape measure and thumb go, now my reply seems like it's been written by an idiot! (Careful)
Hang on. I'll ask the wife.
It'll be fine. It's below the top-tube/seat-tube junction and there's a brace higher up to spread the load.
With that little seat mast, I’d say no.
It's on his shopper. What can possibly go wrong? 😃