Which subject-verb agreement rule applies to this sentence?

When lawrence saw the posters for the circus hanging on the bulletin board, as he swept up the trimmings at slim's barbershop, he knew he would be taking his little cousins. they loved animals, and he was looking forward to seeing the smiles on their faces as the circus performers amazed them with their stunts. but sweeping up hair clippings didn't pay a whole lot. lawrence spent 45 dollars just getting tickets. but it was worth it to see his cousins' faces. the day finally came and everyone was excited as they walked into that big circus tent. but lawrence soon realized that he and his cousins would be thirsty and hungry. as they sat in their seats, the drink vendor walked by selling beverages. desperately thirsty, lawrence asked him how much a lemonade would cost, and the drink vendor said, "eleven dollars each." lawrence replied, "oh, that's it? just eleven dollars? what a great deal for one cup of lemonade. that's totally worth it." the drink vendor walked away. which type of irony is used?