Can we admire the simplicity of the setup? Why FeMC ended up having to give her chocolates 1 day late isn't relevant; the important part is that she was late. It makes you assume that she can make it up tomorrow, no problem (and the second page reinforces that).
Then comes the reveal by the peanut gallery: the childhood friend took initiative and sealed the deal with the FeMC's target on that same night. The one archetype defined by taking their crush's affections for granted and getting BTFO in the common high-school romance plot, SUCCEEDS in this one.
The fourth and final page delivers the coup de grace: FeMC's feelings and efforts have been wasted. There's no convenient consolation prize for her; she has lost. She tries to salvage the situation by eating the ungifted chocolates, but the taste is tainted by the pain of failure.
FeMC has provoked time's wrath, and in turn has witnessed its power.
EDIT: Correction, FeMC actually made the excuse that "since V-Day was not a school day, she could afford to wait 1 day to deliver her feelings". That actually makes her love for her crush questionable: if she had to make an excuse to deliver her love, then perhaps her feelings weren't that strong after all.