Even the Y2K-style ecommerce stores usually had a separate S&H section for some guidance. These days the H part (handling) seems less in vogue (perhaps still common on ebay), while S part is pretty predictable if not free.
It's the T (taxes) part that may be still a tipping point these days, but it's just between vendor and your state,
We are in agreement that there needs to be explanation on what's going on, and not just "we'll set some price yon won't know why".
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.
It's the T (taxes) part that may be still a tipping point these days, but it's just between vendor and your state,