Thanks for the heads up on this.
I might suggest that in addition to purchasing an item from Beretta it would be prudent to let them know why you are purchasing said item. Otherwise the company has no way to separate your actions from somebody who simply purchased something as a matter of routine. For the act of purchasing a Beretta product to be really effective the company needs to understand the connection between your decision to purchase and their positive action of the NRA donation.
There may be a comments section on the Beretta order form, or an even better method would be to take a couple minutes and send them a thank you email via a contact address on their website.
I'm pretty good at letting a supplier know when I'm unhappy with their product or their action, but I'm not so disciplined when it comes to letting them know when I'm happy with their product/action.