That's all right Jack. No point in our doing exams if we can't use the language we learned !
Invitation to treat as what? It's archaic language to describe what is happening ('treat' means ' discuss terms,bargain, negotiate') Anyway, it's clear law. The company's price tag, or advertised price, is only an invitation to you to make it an offer to buy at that price. It can refuse your offer to buy at that price and invite you to offer a higher one. This might be because the first price was shown in error, or because they've decided it is too low now, or because they think you'll pay more, or for no reason at all !