sorry to assume, just wanted to express concern if need be. my apologies!
Shafts should be balanced. By this I mean that you can check balance by sitting in the cart with someone holding the shafts. Put a kid in with you becuase you will be driving with them. Have the shaft holder lift or lower the shafts with you in the cart until they find the spot where the cart balances itself without them having to hang on for dear life.
If that sweet spot is not in the general vicinity of parallel, then you can have problems. Shaft tips too high for sweet spot means that yes, the cart and YOU are doing a pop-a-wheelie! This arrangement pulls up on the horses tummy, very bad.
If your weight is so far behind the axle that the shafts have to be tipped down, that’s bad too. Too much weight on pony’s back.
sometimes in the simple design cart, the seat can be shifted forward or back to change the sweet spot of balance.
NOW - Just because a cart has a sweet spot with shafts level doesn’t mean that the cart fits the pony. Tug loops are meant to have some adjustment up and down to accomodate some height difference BUT!
If you put a cart to a pony who is too tall, the shafts will be tipped up. If the angle of degree is higher than the cart’s original sweet spot, you’re out of balance.
Likewise for ponies too short for the cart.
There are some ways to adjust the HEIGHT of your cart such as addind blocks on top of the axles ( some contraversy to this. See the Line of Draught thread)
Or you can get bigger or smaller wheels to keep your shafts horizontal.
Sometimes the seat can be raised without changing anything else about the cart.
Finding a cart can be as exhausting and frustrating as finding a riding saddle fit.
I suggest again, see this pony driving. If the cart fits, offer to buy. IF they won’t give it up (the cart) , take pics and measurements so that you have info to shop around on. Same with the harness. Do you have an impartial friend who drives who can go with to help you examine all this as you view the pony?