Further research reveals that I have been victimized at least once before by the "higher prices for established customers who buy numerous products in the same category" system at Amazon.com although I dismissed my other mistake as a routine discrepancy.
Forgetting that I had ordered Veronica Mars Season 2 from home already, I placed an order for the DVD set from the store. That meant that I ended up getting two copies from Amazon--one that I ordered at home (where the computer automatically logs me in under my Amazon account, so the computer recognizes me by name) and once at the store (where I "shop blind"—that is, the computer doesn't know who I am until after I've placed the item in my shopping cart, at which point the price is already set).
I reviewed the two invoices, and the price I paid for Veronica Mars Season 2 from Amazon when I ordered under my name was $41.80. the price I paid when I bought the same DVD set "ordering blind" was $37.99. That means that I paid 10% more, apparently because Amazon's sales-tracking software noted that I tended to buy numerous DVD sets of television programs. The system seems skewed to increase prices for frequent customers who make numerous purchases in the same product areas.
So those of you who tend to shop at Amazon.com, be careful... you could be paying far more than you might have to!
(And I checked using the "known shopper" versus "blind shopper" system, and the same Samsung 46" LCD television that was $2699 when I shopped as a known shopper was only $2549 when I shopped blind.)
I have real trouble believing that any of this is accidental...