Shipping looks like a lookup: origin, destination, weight, price. It is not, and the reason is that at the moment checkout needs a number, several of the inputs are still guesses.
What the shop does not know yet
It does not know the real weight. It knows the sum of the product weights, which ignores the box, the padding and the fact that two items ship as one parcel.
It does not know the volume, and for anything light and bulky the courier will charge on volumetric weight instead — a number derived from the box the packer has not chosen yet.
It does not know whether the address is serviceable. Indonesian addresses are not uniformly structured, and the difference between a district that a courier covers and one it does not is often invisible in the text a customer typed.
Every checkout is therefore making a prediction. The question is what the shop does when the prediction is wrong.
Three honest strategies
Charge a flat rate by zone, and absorb the variance. Simple, predictable, and it either overcharges close customers or quietly loses money on far ones — but the business decides which, up front.
Quote live from the courier, and accept the failure modes: an API that is slow at the worst possible moment, and a rate that assumes a weight you had to estimate. Anything live needs a timeout and a fallback rate, because a checkout that hangs on a third party has borrowed that third party’s reliability.
Or confirm before charging. Show an estimate, take the order, and have someone send the final amount after the parcel is packed. Slower, and entirely normal in a market where a lot of ordering already happens as a conversation.
Store what you were told
Whichever route, the shipping figure on the order is a decision made at a moment in time, and it belongs to the order as a stored number — not a recalculation.
Courier prices change. A recalculated shipping cost means a six-month-old order quietly reports a different total than the customer paid, and the first place that shows up is a mismatch between the shop and the accounts.