If you choose to use Social DRM as your preferred DRM setting, you will not be charged the per fulfillment fee.
Adobe DRM and Readium LCP
The "per fulfillment fee" is also called Download fees.
It is a fee charged to you for every downloaded book and is charged on top of your subscription fee. For example, if you sold 50 copies of your book for the month of June 2020 and all of them downloaded the books successfully, then you will be charged 50x$ [per download fee]. It is invoiced to you every 1st of the month granting that your total fee is $10 and above. Otherwise, it gets accumulated and be added to the following month's download invoice.
For more info, also see: