As I have said numerous times in other threads on this topic:

The waiver is tied to the Frequent Flyer number which is linked to the card you have. It does NOT matter what manner if payment you use. I have paid with a 3rd party check, frequent flyer miles, someone elses credit card, someone else' credit card points, another credit card of my own, a United VDB voucher, as part of a Continental Vacation package, as well as the card itself, and all have yielded a free bag without any question.