Do not overthink this. If the CPI has the $5 reimbursement for the second purchase, then it should also say Yes for the required milkshake purchase. The $17.75 maximum reimbursement is subject to the additional milkshake requirement. Sometimes they modify a listing to remove the milkshake but forget to modify the maximum reimbursement. The CPI will clearly show the fee and the required first purchase, in most areas it is now $12.75 maximum.
The golden rule is to check your CPI and guidelines just before the shop. Not 24 hours before, not when you assigned the shop, but just beforehand. Most MSCs warn you to check for updates just before performing a shop as they are subject to changes after being assigned.
My posts are solely based on my opinions and for my entertainment, contact a professional if you need real advice.
When you get in debt you become a slave. - Andrew Jackson