I see two items for $5, but when I add the imported one, suddenly it costs more — and Amazon didn’t tell me that ahead of time or give me any way to choose the one without tariffs on the grid/list view.
This makes tariffs more effective because they can’t bump the domestic price to match — while giving customers a negative chock each time they choose an importer for a product.
This is were tariffs can go horribly wrong: it destroys the incentive for competition between companies.
They'll just raise the price of the domestic good to $13 and we will all pay $8 extra on a thing that used to cost $5.
If the price displayed is still $5 but tariffs added at the end, the domestic seller's $13 sticker price will not look attractive to buyers.
For goods that have alternatives, businesses may choose to under-price (relative to their tariffed competitors) in order to gain sales and customers.
The choice for consumers won't be "choose between a $5 item and $15 item" it will be "choose between $13 and $15", like I mentioned above.
This doesn't work as easily if the sticker price for the imported good is $5 and the real price displayed at the end of the purchasing funnel. The local business will have to keep its sticker price at $5 to avoid losing customers when they initially compare goods or rely on customers to come back to them once they get faced with the tariff tax, which will also lose customers.