1\section{Policy and practical implications}\label{sec:policy}23The results are descriptive and the window is short, so implications are stated as what the evidence supports and what it would take to go further.45\emph{For consumers and comparison services.} On identical products the choice of banner moves the posted price by 10--25 log points, and a shopper restricted to two banners captures most of the attainable posted-price saving. A comparison service that reports the \emph{weekly modal} price, flags multi-price weeks and labels same-company pairs would convey this architecture more faithfully than one that reports the last scraped price.67\emph{For statistical agencies and researchers using web-scraped prices.} Store-selected storefronts are a mixture of store prices unless the collector pins the store. The Billion-Prices-style assumption that one online price represents the retailer does not hold for Quebec's large chains; high-frequency ``changes'' in such feeds overstate price flexibility by an order of magnitude, and the correct unit of observation is a store-identified session or, failing that, a weekly modal price with the range reported.89\emph{For competition policy.} The like-for-like evidence is overwhelmingly \emph{within} company: the Loblaw and Metro discount banners price 10--18 log points below their conventional banners on the same UPCs. This is consistent with deliberate format segmentation inside concentrated groups, which may be efficient (serving heterogeneous consumers) or may be a substitute for cross-chain competition; posted prices cannot tell these apart. Cross-chain exactly matched comparisons are rare in this source because chains differentiate assortments and private labels, which is itself a finding about how competition operates.1011\emph{For the measurement of food inflation.} Over eleven weeks the matched-model storefront index and the naive average agreed at the market level, but naive averages for individual banners were off by 10--30 points because of assortment turnover. Any index built on catalogue data must be a matched-model index; any claim about ``shrinkflation'' requires storing package labels over time, which this source did not do and which is a one-line change in the collection design.12