As a developer I would approach this differently. Assuming I have read the question correctly...maintain a list of categories that are allowed to be in the "£80" special. When customer adds items to his basket, do a check to see if either are in the "£80" list. If yes, show special button, otherwise show standard button. E.g. Specials Event 1 Event 2 Event 5 Event 6 Event 12 Customer adds Event 1 and Event 2 to basket = Special rate Customer adds Event 1 and Event 3 = Standard button
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