Delineate their options clearly
As others have noted, you need to separate this in the UI. You're only creating "friction" that will inhibit your conversion rate. You have two customers to deal with in this scenario:
Customer A: Just wants to place an order. Let them do it smoothly.
Customer B: Wants to communicate a preference. Let them understand what to expect when they chose this option.
Don't try to change behavior
Most online customers (the vast majority) don't want to talk to you. Let them contact you through the site. Offer the option to make their request via chat or email or an inline form. You can easily report back to MI with this data and it will probably be richer than the trick the business wants you to play on them.
