Collect shipping option for select customers

I have always offered customers the option to include their own UPS account number and we would ship the order collect to their account. Previously I used offline credit card processing so I would just removed the shipping line before running the card if they provided a UPS account number. I have switched to having the site process the credit card when the order is placed. I am wondering if there is a method I could use to flag a customer or set up a group that will either always get free shipping or give them an option to choose ship collect. I do not want to just offer a collect shipping field to everyone and them have people choose it for free shipping with a bogus account number. I am ok with having the customer either enter the account number every time or having them provide it to me when I flag them as a collect customer. Any suggestions would be greatly appreciated!