Is it fine to use 2.4 GHz nRF24l01 modules in commercial products under FCC rules?
2.4 - 2.4835 GHz is unlicensed indoors or outdoors. The one caveat is the maximum transmit power allowed fed into an antenna is 30 dBm (1 Watt). The maximum EIRP is 36 dBm (4 Watts). The max output power from the nRF24l01 is 0 dBm so this will not be an issue.