These three letters are filled to the brim with joy and hope and promise. Celie tells God that she finally holds a letter from Nettie. Nettie's letter with the funny-looking stamps is proof that not only is Nettie alive, but that Celie's babies — Olivia and Adam — are also alive and are "coming home before the end of another year." Celie's joy, you should note, is doubled because she has someone to share it with: Shug.
We realize that Nettie, although she is now a missionary, is not and never was as naive as Celie was. She is aware that Albert has probably been hiding her letters from Celie, but she counts on the sentiment associated with Christmas and Easter to perhaps soften his fierce resolve to punish both her and Celie by keeping her letters from Celie.
How do we know that Albert has been deliberately cruel and vindictive for all these years? From two things that Celie says, and Celie doesn't lie. Initially, she tells Shug that Albert couldn't have kept Nettie's letters from her because he knew that Nettie "mean everything in the world to me." But later, she tells God that she finds the letters in Albert's trunk, and everything that mean something to Albert go in his trunk. Clearly, Albert has structured his life around deliberately punishing both Nettie and Celie — for Nettie's rebuff long ago and for Celie's not being either Nettie or Shug.
Celie's normally gentle character is transformed immediately. Even she herself realizes that she's acting "just like Sofia," muttering and "crazy for Mr._______'s blood." And now that Celie is strong enough to listen to Shug's evaluation of Mr. ________, Shug explains that Albert wasn't always as mean and callous as he has been to Celie, but that he used to dance, laugh, and make her feel great. Shug's affair with Albert "had to be good," Shug explains, or otherwise she could never have loved him. Albert was Shug's first source of unlimited affection; she was reared by a reserved mother and an inhibited father. Shug was hurt when Albert married Annie Julia — and that was part of the reason why Shug treated Celie "so mean. Like you was a servant." It was after Annie Julia's death that Albert changed.
Now that Celie knows about Shug's relationship with Albert, and Shug knows about Celie's background of rape and mistreatment, the two women become even closer, especially so since Shug found one of Nettie's letters in the mailbox and, along with Celie, she was determined to find the rest of Nettie's letters.
Celie is reunited with Nettie, even though they are continents apart. The person who meant more to her than anyone else in the world is alive. Despite the fact that Celie never received any letters from Nettie, she never stopped loving her and never became bitter about the fact that she never received any letters. In fact, Celie's constant love for Nettie has been a source of sustaining strength for her, and now that Nettie is alive, Celie's self-confidence becomes so strong that she will frighten the men who have made her cower for so long. Celie takes the strength of Sofia and the spunk and sass of Shug, and she draws on her own awakened, rightful sense of vengeance to emerge a solid, independent, courageous, and admired woman.