My guess is that they (Best Buy) cross referenced the name they read from my credit card to one of the bulk mail lists they purchased for marketing purposes. The letter was addressed to me 'or current resident' and inside was information about how my player with this new firmware update could download Netflix movies. The update CD itself was for my specific model (BD-P2550).
The other possibility is that they cross-referenced my in store purchase via the card number to a previous on-line purchase from their web store (which would have included a shipping address). In either case, the mag stripe of my card (in an otherwise anonymous transaction) was used to make the connection, and four months later a package with a firmware update arrives at my house.