If your planning on going the cheap route first, you can get regular plastic shelves and change the distance between shelves by cutting the spacer posts. Or you could just start stacking your tubs without a shelf.
It may be a bit difficult to get husbandry right though. I wouldn't go cheap on a thermostat though if I were you.
You should not use those. I thought of it too a long time ago. When you pull open an upper self, the bottom one it exposed which means three outcomes. Nothing happens, the ball python escapes, or he is attempting to escape and you close the top one and squish.
Also they are very flimsy. Any size snake can push through them without effort.
Someone asked this on the caging forum not long ago. These suck. I own two of them and they're incredibly flimsy, have nowhere to attach heat tape to except the bottom of the tub itself, are poorly sized for most life stages of balls, and there is very high risks of breakouts for basically any activity (including leaving them alone).
Not worth it, buy an actual rack or build your own.
I owned one for a while and used it as a cheap dresser. Fell apart within a month. I wouldn't house any animal in one of these. There are plenty of blueprints int he DIY section that are cheap to produce, you just have to be willing to put in a little effort into construction. The simplest thing you could do is buy a metal shelving system, line heat tape along the back of the shelves and buy tubs that will fit on the shelf with locking tops, drilling holes for air.
"Your absence has gone through me like thread through a needle. Everything I do is stitched with its color."