It's the T (taxes) part that may be still a tipping point these days, but it's just between vendor and your state,
In my experience, the most fluctuations were on international shipping by small vendors. Lego bricks for instance, where it makes a big difference if you request 5 small pieces that weight 20g total and can wait 3 months, or if it's 500+g in a middle sized box and you want it in 2 days.
Even with average indication on what to expect, depending on the combination you are requesting the vendor might use a different carrier, different shipping method and so on. They could make it more simple with a range of arbitrary standard fees, but then it costs a lot more to the customer, putting the vendor at a disadvantage price wise. In particular people have visceral reactions to overly high shipping prices.
There’s a lot of legitimate case were showing shipping price upfront is just not doable or valuable to the customer.
BTW there are a surprising amount of shops for specialized goods that won’t even list the final price at the end. The customer places an order, and they update it with a finalized price after a human looks at the content, and from there the customer is free to pay the transaction or give up the order.