Hello there. If anyone could help with the following contract law question, it would be greatly appreciated.

Timbo's Caf operates a loyalty scheme of a type common in cafes across Sydney. Customers are free to take a 'Loyalty Stamp Card' from the counter at the front of the caf, and to have it stamped each time they purchase a coffee or other beverage. Each 'Loyalty Stamp Card' bears the following notice:

'Collect ten stamps and get your next coffee free!'

Mark collects a card on his first visit to Timbo's Caf and has it stamped when making each of his next ten coffee purchases. On his eleventh visit, he proudly presents the coffee card to the cashier and requests his free coffee, but is told that he is not entitled to one as the loyalty scheme has been discontinued. The cashier directs Mark's attention to a sign just below the register which announces this fact. The cashier explains that the sign announcing the discontinuation of the scheme has been on display since the previous week. Mark realises that the sign must have been on display when he bought his ninth and tenth coffees, and that it has escaped his notice.

Advise Mark as to any action he may have against Timbo's Caf.

I have considered the possibility of the coffee card being a conditional gift or a unilateral offer but can't quite pin point which? I've also looked at cases such as AUS Woolen Mills, Carbolic Smoke bomb and Chappel and Co v Nestle to figure out whether there is an agreement and consideration, but some clarification and guidance would be great.