~snip~
You are basically correct, yes : )
The transaction fee, if its needed (if the trading profit will not cover it), will be billed to the vendor, much like currently mastercard and visa act.
Vendor as in to the owner of the store where the buyer buy the items or as in the manufacturer of the items?
The vendor, as in the store who sells the items, yes : )
How do you plan to keep the vendors from billing the buyers of those transaction fees? It's not an impossible scenario where the sellers decide to reap better profit by subjecting those (or any fees involved) to the buyers by increasing the product price.
This would only be an issue, if the cryptopayments were more expensive than fiat ones but since cryptopayments are cheaper (no middle-men such as mastercard or visa taking their share + the upkeeping of POS bank terminals) there will be no reason for the sellers to add extra price to the products. To be fair, for those vendors who do not wish to receive cryptopayments directly, form them Smarts will be the middle man who takes care of crypto and will return fiat to the seller. But even those costs will be less than half of what they are paying right now.
So in a nutshell, Vendors will have no need to increase the pricing.