(CBS SF) — Airline fare wars are making a comeback, and that’s good news for passengers everywhere.

A number of cities are seeing low prices this week after four airlines launched sales with unbeatable deals — but only if you’re flexible on seats and dates.

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Frontier is offering flights as low as $15 — you read right — $15! Most flights have to be booked by the end of August, but can take you from Phoenix to Chicago, Denver to Portland (Oregon) and other destinations for $19 if you fly on Tuesday, Wednesday and sometimes on a Saturday. Escape to Phoenix from San Francisco for $29 if you fly this week or $69 if you opt for a later time in between now and November 18.

Southwest has $82 flights from Oakland to Seattle and San Francisco to Atlanta for $156. The cheapest $39 fares features one-way routes between Washington, D.C. to Akron-Canton, Ohio or Indianapolis.

Spirit Airlines’ special fares start at $34.10 one-way for routes like Atlanta to Boston, Baltimore to Chicago, Oakland to Chicago, Dallas, Houston and Las Vegas. Knock off $10 if you’re part of the airlines’ $9 Fare Club Member.

Jet Blue’s promotion is over, but featured flights starting at $49 from Boston to Richmond and $99 on routes from New York JFK to Jacksonville.

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For finding a bargain fare on any of the above airlines, skip flying on Friday or Sunday and opt for midweek travel during the off-peak fall season.

“This is the big break consumers have been waiting for in response to lower fuel prices,” says Seth Kaplan, managing partner of industry newsletter Airline Weekly.

The price of oil is at the lowest level in six and a half years and the industry is saving billions of dollars on fuel, giving airlines leeway to cut fares but still post healthy profits. Airlines have also added larger, more efficient planes to their fleets while packing more seats into existing jets. So, while the number of domestic flights is down slightly over the past 12 months, there are now 3.4 percent more seats for sale — too many to meet the demand in all cities. To fill those extra seats, airlines have had to offer discounts.

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