In the first episodes of You season 3, Joe Goldberg (Penn Badgley) follows through on his promise to send Ellie Alves (Jenna Ortega) money, only to seemingly forget about her later on. Even when Joe settles down in Madre Linda with Love and their son Henry, he still recognizes his obligation to Ellie, but with limited resources, he has to resort to another means of acquiring money, which then paved his way to meet Marienne (Tati Gabrielle). However, after securing a job at the local library, Joe seemingly forgets about Ellie. And though it may appear to be a convenient plot hole, it's actually a reflection of his season 3 development.
Following the events of You season 1 finale, Joe — now identifying as Will Bettelheim — moves to Los Angeles in search of a clean slate and meets Ellie and Delilah Alves (Carmela Zumbado), who eventually got caught in Love Quinn’s (Victoria Pedretti) manipulations. Joe's decision to save Ellie and send her away before Child Protection Services caught up with her and put her through the same system that Joe counted himself a victim of was fitting considering his past. And in the warped morality of Joe's mind, his almost paternal instincts for Ellie were a sign of the remnants of humanity still within him.
One of the central themes in You season 3 is Joe’s drive to be better, and while that includes rejecting the next-door neighbor and straightening Love’s messes, it also involves fulfilling his promise to Ellie. In the several months since Joe arrived in the suburbs, he seems to have no difficulty honoring this commitment and continues to send her money. But as he grows tired of the suburban dress-up his fake happy life with Love represents and the monster within him grows again, Joe falls back to his patterns. As part of that regression, which Joe sees as his embracing his personal freedoms (of expression and identity), it quickly becomes evident how easily he can turn his back on Ellie and his obligations to send her money disappear out of You season 3's story.
Despite not being directly involved in Delilah’s murder, Joe felt guilty for taking away Ellie’s role model and guardian and sees in her a potential echo of his own past in the system. Consequently, sending Ellie money is his way of clearing his conscience, as well, of course, as satisfying Joe’s need to protect women, which he no longer believes Love can provide, given her volatility and wilfulness. However, in You season 3, when he finds another target in Marienne, he completely embraces his dark side again and changes his priorities and it's no surprise that Joe forgets to send Ellie money. He simply has no time nor inclination to steal and repair rare books for money because he is busy building a future with Marienne, who would be a victim of the theft anyway. Whatever nice guy facade he is supposed to be maintaining is not important anymore, since underneath it lies the same disturbed monster who only does things for his own benefit.
Joe may have lost his shot at becoming a good person by ignoring his monetary obligation to Ellie, but that is a fitting reminder that he is now beyond redemption. Unfortunately, Ellie is just a pawn in his attempt to atone and when he forgets to send her money, he's showing his true colors. Wherever he goes, Joe finds another target to be intoxicated with, and no amount of guilt is enough to mask that need, especially since he always gives in to it. At this point, time will only tell if this cycle still continues in You season 4.