Why can’t I get shipping estimates right??

I recently upgraded to Pro, so I made a shipping template. Basically, I set it up with $0 base fee (maybe that’s my mistake), + the price I found online per pound. I make sure I weigh and measure the item ahead of time so that the estimate is based on the actual weight and dimensions. I still end up getting shipping costs several dollars above what the buyer paid. Is it because the cost per pound changes? Or having a $0 base fee? Is there some strategy for more accurate shipping I’m missing? It’s ALWAYS more expensive than what the buyer paid. I’m losing a decent amount of money on shipping.

TIA