Best answer: You may be able to call the location you were planning to pick it up from, and ask an employee if they have it in the store and if so, to hold it for you for the day (in my experience I wouldn't expect any longer than that). In the event they don't even have it available at said location, I am also unsure if they would be willing to ship that item to the store so that you would be able to then purchase it there, but it couldn't hurt to ask.
Otherwise, through the website (as far as I can tell) there is no way for you to put in an official store-pickup order where you don't pay until you pick it up. (What would be the effective difference of you just coming into the store and then purchasing it right there and then?)
In my experience, calling a particular Best Buy location will direct you to an automated menu that won't tell you exactly how to get ahold of an in-store employee, so if that happens I'd just press 0 a couple times until you do. (This has worked for me, especially if they're asking for an extension # that wouldn't be obvious to you)
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