"We'll donate a slice for every pie we sell to local food banks system wide."
So when you gett your pizza it's missing a slice?
Sounds like a way for the pizzeria to take your missing slice and resell it as 'by the slice' to sumbuddy else.
Fortunately, I like anchovies on pizza - good luck to them trying to resell the missing slice on that order. It'll be sitting under the heat lamp for a week.
Mebbe they wholesale all the individual slices to Sbarro - nobody would notice the anchovies.