Final answer:
Mark a 'D' or 'Decaf' on the cup for a decaf shot, and '½ Decaf' for a half-decaf espresso shot to indicate the customer's preference.
Step-by-step explanation:
If a customer wants a decaf or half-decaf espresso shot, you will mark their cup to indicate their preference. Usually, baristas have a set of abbreviations for different drink orders. For a decaf shot, you might mark the cup with a 'D' or 'Decaf'. If the customer wants half-decaf, you could write '½ Decaf' on the cup to show that they want half of the espresso to be decaffeinated and the other half to be regular.