Hi
Tennielle Crook,
What you need to get is a Copyright Licence Agreement, which sets out the terms upon which you are able to use and reproduce the artists artwork.
The essential components to this agreement are: (1) the granting of an unlimited or restricted/specific licence to use the artwork and (2) consideration or payment for the use of such (3) the term or duration of the agreement.
There are no hard and fast rules on what you need to pay or how you need to pay, you can come to whatever agreement you like. You may choose to structure it on a retainer fee agreement - whereby you reproduce as much as you like for a specific purpose during a one year period for a yearly fee, or alternatively as you said, you could agree to a per copy price.
There are probably templates of this type of agreement out there somewhere on the web, which may give you some ideas as to things you need to incorporate in it besides the things mentioned above.