USA Medical Devcie Excise Tax Refund

[Russ Dostilio]

We are a medical device manufacture that paid the 2.3% excise tax on total sales price. We should have used the lower"constructive sales price" to calculate the tax. We are preparing to request refunds for the past three years. We no longer have to file the 720 Quarterly Federal Excise Tax Return. Do we file amended returns - Form 720X? Or do we file Form 8849 and Schedule 6? Our research and interpretation of the IRS publications have confused us and led us into conflicting opinions of how we should proceed.
Thanks for all your help.
Russ