Standardizing Thresholds Of Penalties for Fentanyl Act or the STOP Fentanyl Act This bill reduces 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. The bill also creates enhanced criminal penalties for certain violations involving fentanyl that was imported along the U.S.-Mexico border. Specifically, the bill reduces from 400 to 5 grams the fentanyl quantity and from 100 to 0.05 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 0.5 grams the fentanyl quantity and from 10 to 0.005 grams the fentanyl analogue quantity that trigger a mandatory minimum prison term for low-level first-time or repeat offenders. Additionally, the bill creates enhanced mandatory minimum prison terms for importing or exporting fentanyl that was imported along the U.S.-Mexico border.