If your products are downloads, you could just use "Send Free Download Link" in your Seller Admin to manually issue each affiliate a link to obtain their review copy at no cost.
If you'd prefer to use the discount method, you could create a secret discount code for each affiliate, which marks down the item price to 0.00 (either a 100% All Cart Items discount or a product-specific discount equal to the full item price); you would probably want to set Max. Use Count = 1 for these, to make sure nobody else could use the code after the affiliate has used it once (e.g. in case they try to share their code). This help page explains our discount settings in detail: