that's true that the calculations can get complicated pretty quickly in ecommerce, but google probably has all the data it needs (origin zip, destination zip, likely carrier(s), possibly even the weight/size of each item) to provide a pretty good estimate in most cases. they could even calculate a range for [1 item per box, all items in 1 box].

the important bit is to present it as a separate line item (with grand total) so that consumers can decide how much to trust the estimate.

that would be an even clearer shot across the bow of amazon, walmart, and the like, who provide comparison across their own platform merchants, but not across all merchants everywhere.