Gas for the car? Cheaper via cash. This becomes all the larger when gas prices are higher.
The credit price is about 2% higher, but I get 3% credit card rewards on fuel purchases. So I come out ahead paying by credit, and save time, too.
Keep in mind that Amazon still doesn't collect tax for out-of-state third-party sellers (unless the order is "Fulfilled by Amazon"). From your Amazon account page, you can download your annual purchase history in spreadsheet-compatible format, with the taxes broken out into their own column, and use that to figure what purchases still need to be accounted for for calculating use tax. That was more necessary back when they didn't collect any California sales tax at all, but still comes up if you're buying from third-party sellers.
Note that if you haven't kept receipts to calculate your use tax, you're supposed to estimate it as (for 2013 in California anyway) 0.033% of your adjusted gross income. Depending on your spending, that works out to being as if, very roughly, 2% of your spending were out-of-state.
Can't tell if sarcasm; ITA Matrix is one of the most flexible and powerful flight search engines out there.
It's kind of useless to search for the cheapest fares in a whole year, because seat sales are often offered a month or two before the flight, not to mention most airlines only sell seats up to 11 months in advance. Use something like Airfare Watchdog if you want to hear when seat sales happen, then use ITA Matrix to pick dates and routes in the timespan that the seat sale is in effect.
ITA Matrix also has an advanced routing language, which you can use to specify alternate airports, restrict what carriers will be used, what airports to connect through, how many connections to make (both minimum or maximum), what fare codes to use, etc.. I haven't yet found anything else like it on the web; I use it all the time to find flights that will maximize my frequent flyer mileage and minimize my cost.