It’s more of a cop-out answer. The reality is it takes extra design, logic validation, business model validation, dev, QA, support, etc. in multiple cycles to add more functionality. Lots of features, and not enough man-power to get it all done. Life as usual for any dev house. So it becomes someone’s job to get impatient customers to simply accept the current limits.
While much of the accepted answer’s limitations do apply to many use cases, it does not apply to all. For example, we’re trying the “just markup the products” approach, which we can’t continue if we’re expected to price match other vendors online. For some items, Shopify has actually disabled the items because it violates price gouging policies, when all we’re doing is exactly what the accepted answer suggested! (Damned if you do, damned if you don’t, sigh). We also can’t use the “just use a flat shipping rate” approach, because we already use that for another purpose.
The only correct way for our business model to handle this is to leave the product price exactly as it is on other sites, leave the shipping rate as it is, and add a variable service fee (which is basically a % applied to selected products). However, Shopify does not offer the mechanics (or at least none I’ve found yet on my plan) to do this. Per the API, /cart/update is limited to “note, attributes, or line item quantities”, /cart/change is limited to qty, and /cart/add requires an existing variant_id. So there is absolutely no way I can add a custom item on the fly with my precise unit_price, or adjust an existing variant_id to update the unit_price for a cart.
It’s possible I’ve just outlined an edge case, aka “low priority”, aka “will never get done”.