I am a Store Manager of a Retail store and can tell you the bike shop would not have a chance in hell of winning in small claims court. I do not care how long it has been, you ordered something from experts and they even wrote on your receipt it was 19, their mistake, therefore, they have to fix it. It is not your job to double-check, it is theirs. Every business knows that on some customers you lose money due to mistakes, that will remind them to not make that same mistake again. That being said, I would approach it in a nice manner, see what they will do, and if they do not satisfy you go to small claims court. They will lose, I lose all the time when I have paperwork that says the customer got exactly what they paid for and signed for it. The judge always favors the consumer. Good Luck