What is a "per fulfillment fee"?

Social DRM

You will not be charged the per-fulfillment fee if you choose to use Social DRM as your preferred DRM setting. 

Adobe DRM and Readium LCP

The "per fulfillment fee" is also called the Download fee. 

The per-download fee is an additional charge you must pay every time someone downloads your book. It is on top of your subscription fee to use our service.

For example, if you sold 50 copies of your book monthly and all 50 people downloaded it, you will be charged 50 times the per download fee. The charge is invoiced to you on the 1st of every month, provided your total cost is $10 and above. If it is less than $10, it will be added to the next month's download invoice.