A small apothecary opened in my neighborhood that makes beauty products. They’ve been very accommodating with selling me shampoo and body wash in my own reused containers. It’s all locally sourced ingredients. I’m new in my Zero Waste journey but it feels great to eliminate plastic from part of my daily routine.
So the common plastic pump you see in every soap dispenser has two parts - the part you screw onto the soap container, and the piston thingy, above that, that you push down to get the soap to spurt out of and into your hand.
If you put something above the screwy part, on the lowest part of the piston, you reduce the amount that the piston goes down. And reduce the amount of soap that can come out.