A cheap McDonald’s order can quickly spiral past $20 when delivery fees pile up, and customers are erupting over what they say is an avalanche of charges that undercuts the chain’s promise to offer value. Diners took to social media to blast the fast-food giant and its delivery partners over a growing list of add-on costs. Those include delivery fees, service fees, menu markups and — perhaps most galling of all — a controversial “small order fee” that kicks in when customers don’t order enough.