Ending the Fentanyl Crisis Act of 2021 This bill modifies the drug quantity thresholds that trigger a mandatory minimum prison term for a defendant who manufactures, distributes, imports, exports, or possesses with intent to distribute fentanyl. Specifically, the bill reduces from 400 to 20 grams the fentanyl quantity and from 100 to 5 grams the fentanyl analogue quantity that trigger a mandatory minimum prison term for high-level first-time or repeat offenders. It also reduces from 40 to 2 grams the fentanyl quantity and from 10 to 0.5 grams the fentanyl analogue quantity that trigger a mandatory minimum prison term for low-level first-time or repeat offenders. Additionally, the bill directs the U.S. Postal Service to increase the availability of chemical screening devices and dedicate the appropriate number of personnel to interdict fentanyl and other substances that are unlawfully imported into the United States.