Lavigne's third album, The Best Damn Thing, was released in April 2007 and debuted at number one on the U.S. Billboard 200, and subsequently achieved Platinum status in Canada. The album sold more than 2 million copies in the US. Its lead single, "Girlfriend", became Lavigne's first number-one single on the U.S. Billboard Hot 100 and one of the decade's biggest singles. The single also peaked at number one in Australia, Canada, and Japan, and reached number two in the UK and France. As well as English, "Girlfriend" was recorded in Spanish, French, Italian, Portuguese, German, Japanese, and Mandarin. The International Federation of the Phonographic Industry ranked "Girlfriend" as the most-downloaded track worldwide in 2007, selling 7.3 million copies, including the versions recorded in eight different languages. "When You're Gone", the album's second single, reached the top five in Australia and the United Kingdom, the top ten in Canada, and the top forty in the US. "Hot" was the third single and charted only at number 95 in the US, although it reached the top 10 in Canada and the top 20 in Australia.
Lavigne won two World Music Awards in 2007, for 'World's Bestselling Canadian Artist' and 'World's Best Pop/Rock Female Artist'. She won her first two MTV Europe Music Awards, received a Teen Choice Award for 'Best Summer Single', and was nominated for five Juno Awards. In December 2007, Lavigne was ranked number eight in Forbes magazine's list of 'Top 20 Earners Under 25', with annual earnings of $12 million. In March 2008, Lavigne undertook a world tour, The Best Damn World Tour, and appeared on the cover of Maxim for the second time. In mid-August, Malaysia's Islamic opposition party, the Pan-Malaysian Islamic Party, attempted to ban Lavigne's tour show in Kuala Lumpur, judging her stage moves "too sexy". It was thought that her concert on August 29 would promote wrong values ahead of Malaysia's independence day on August 31. On August 21, 2008, MTV reported that the concert had been approved by the Malaysian government.
Lavigne possesses a soprano vocal range. Themes in her music include messages of self-empowerment from a female or an adolescent view. Lavigne believes her "songs are about being yourself no matter what and going after your dreams even if your dreams are crazy and even if people tell you they're never going to come true." On her debut album, Let Go, Lavigne preferred the less mainstream songs, such as "Losing Grip", instead of her more radio-friendly singles, such as "Complicated", saying that "the songs I did with the Matrix ... were good for my first record, but I don't want to be that pop anymore." Lavigne's second album, Under My Skin, had deeper personal themes underlying each song, saying that "I've gone through so much, so that's what I talk about ... Like boys, like dating or relationships". Musically, it's considered her heaviest album that showcased ballads and rocker songs with a more post-grunge sound that verged on nu metal, which anticipated the emotional intensity and theatrical aesthetics of emo-pop music before it came into the mainstream. In contrast, her third album, The Best Damn Thing, was not personal to her. "Some of the songs I wrote didn't even mean that much to me. It's not like some personal thing I'm going through." Her objective in writing the album was simply to "make it fun". The style has been praised for her ability to combine bubblegum pop melodies with punk rock riffs. Goodbye Lullaby, Lavigne's fourth album, was much more personal than her earlier records, with Lavigne describing the album as "more stripped down, deeper. All the songs are very emotional". Ian McKellen defined her as "a punk chanteuse, a post-grunge valkyrie, with the wounded soul of a poet and the explosive pugnacity of a Canadian" on The Late Late Show with Craig Ferguson in 2007.
In 2007, Chantal Kreviazuk, who wrote with Lavigne on her second album, accused Lavigne of plagiarism and criticized her songwriting, saying that "Avril doesn't really sit and write songs by herself or anything." Lavigne also disclaimed this, and considered taking legal action against Kreviazuk for "clear defamation" against her character. Kreviazuk later apologized, saying that "Avril is an accomplished songwriter and it has been my privilege to work with her." Shortly after that, Tommy Dunbar, founder of the band the Rubinoos, sued Lavigne, her publishing company, and Lukasz "Dr. Luke" Gottwald for allegedly stealing parts of "I Wanna Be Your Boyfriend" for her song "Girlfriend". Gottwald defended Lavigne, stating, "me and Avril wrote the song together... It has the same chord progressions as ten different Blink-182 songs, the standard changes you'd find in a Sum 41 song. It's the Sex Pistols, not the Rubinoos." In January 2008, the lawsuit was closed after a confidential settlement had been reached.
In mid-2007, Lavigne was featured in a two-volume graphic novel, Avril Lavigne's Make 5 Wishes. She collaborated with artist Camilla d'Errico and writer Joshua Dysart on the manga, which was about a shy girl named Hana who, upon meeting her hero, Lavigne, learned to overcome her fears. Lavigne said, "I know that many of my fans read manga, and I'm really excited to be involved in creating stories that I know they will enjoy." The first volume was released in April 2007, and the second followed in July 2007. The publication Young Adult Library Services nominated the series for "Great Graphic Novels for Teens".
Lavigne has been involved with many charities, including Make Some Noise, Amnesty International, Erase MS, AmericanCPR.org, Special Olympics, Camp Will-a-Way, Music Clearing Minefields, US Campaign for Burma, Make-A-Wish Foundation and War Child. She has also appeared in ALDO ads with YouthAIDS to raise money to educate people worldwide about HIV/AIDS. Lavigne took part in the Unite Against AIDS concert presented by ALDO in support of UNICEF on November 28, 2007, at the Bell Centre in Montréal, Québec, Canada. In November 2010, Lavigne attended the Clinton Global Initiative.
Lavigne worked with Reverb, a non-profit environmental organization, for her 2005 east coast tour. She covered "Knockin' on Heaven's Door" for War Child's Peace Songs compilation, and she recorded a cover of the John Lennon song "Imagine" as her contribution to the compilation album Instant Karma: The Amnesty International Campaign to Save Darfur. Released on June 12, 2007, the album was produced to benefit Amnesty International's campaign to alleviate the crisis in Darfur.