For salads, sandwiches, snacks, etc., go to the streetside cafés. You can still have a light lunch there for 10 euros ($9.40) unless you are in the heart of a tourist area, where it will cost twice as much.
In lieu of a sales tax, France has a value-added tax (VAT, or TVA in French) which is 20.6% for most things. Prices are always inclusive of VAT, and they also include 15% service in restaurants, bars, etc. (remember that when comparing prices with the U.S.) Therefore tips are unnecessary, as well as uncommon. Taxi drivers still tend to expect some "rounding up" of the fare, but you have the right to pay only the meter amount. Note however that there are surcharges for pick-up at airports and train stations, and for luggage. These are explained, in French and English, on a decal on the rear left window of the taxi.
If you buy an expensive item to take back to the U.S., ask for a "detax" voucher from the merchant (a minimum amount is required: grouping your purchases may help reach it; this amount changes from time to time and depends on the type of item, so you should ask the store clerk first). When you leave France, carry the merchandise in question with you instead of placing it in a checked bag. There is a customs counter just before passport control at the airport, where upon presentation of both the voucher and the merchandise, the agent will stamp the voucher to certify that you have indeed exported the item. Mail the voucher back to the merchant to receive a refund of the VAT you paid. The merchant usually provides you with a self-addressed envelope to do this, and there is a mailbox in the terminal for that purpose after passport control; some customs desks will also take the envelope and mail the voucher for you. If you paid by VISA, the tax amount will be credited back to your VISA account, within about four to six weeks.