When the summer rolls around, what lucrative venture, ironically speaking, do we find Frieda and Claudia engaged in?

In April, Claudia and Frieda order marigold seed packets. They plan to sell them for five cents each so they can buy a bicycle. The packets don't arrive until "the summer was already thick."
Their mother says they can only sell to people who she knows or to people in the familiar neighborhoods. However, Claudia seems to indicate that they went into other places to sell as well. That summer, the two girls were only focused on selling the seeds. They were so preoccupied to it that she says they only listened with half an ear to things that people said to them.
Traveling to different houses to sell seeds is how they start to understand what's going on with Pecola and her baby. The two girls end up planting their seeds and hoping that Pecola's baby will live because of their actions; however, the marigolds never grow. Toni Morrison writes:

What is clear now is that of all of that hope, fear, lust, love, and grief, nothing remains but Pecola and the unyielding earth. Cholly Breedlove is dead; our innocence too. The seeds shriveled and died; her baby too.

The two girls give up the idea of the bike to try to help Pecola but ultimately her baby doesn't live, they never get their bike, and the marigolds never grow either.

At the beginning of chapter 10 in The Bluest Eye, Claudia explains how she and Frieda had begun a fundraising project at the beginning of the summer so that they could buy a new bicycle.
Having waited since April to receive their package, the girls begin selling packets of marigold seeds for five cents. Claudia explains from her mature perspective that they “believed it.” This indicates that the marigold seeds were indeed not a profitable endeavor, but at the time, Claudia and Frieda were fooled into believing they would make the money they needed for the bicycle.
Claudia and Frieda traipsed all over town trying to sell their seeds, but Claudia does not mention a single sale in the paragraphs that follow. Instead, Claudia and Frieda earn access to the story of Pecola’s incestuous pregnancy. The adults in town gossip about Pecola, blaming her for the pregnancy and expressing the idea that it would be better if the unborn child died.
Claudia and Frieda are horrified to learn that no one thinks Pecola’s baby should live. To counteract this, Frieda suggests that they bury their seed money (“two dollars”) and plant the remaining marigold seeds as a sacrifice to God. They hope that by giving up something they want (the bicycle), their prayers to let Pecola’s baby live will be answered.