FedEXによれば小さなバッテリーは制限があるものの送ることは可能なようです。
https://www.fedex.com/content/dam/fedex/ca-canada/MVP/images/2020/Q2/FX_HowToPack_Batteries_CA_EN_401131600.pdf
Shipments of lithium metal batteries packed with equipment and lithium metal batteries contained in equipment accepted by FedEx Express may be packaged in accordance with Section II IATA packaging requirements provided that the metal or alloy cell content does not exceed more than 1g, and the aggregate lithium content does not exceed 2g per battery.
Shipments of lithium batteries packed with equipment and lithium batteries contained in equipment accepted at FedEx Express must meet the general requirements of the packaging instruction. For lithium ion or polymer cells, the watt-hour rating is not more than 20Wh and 100Wh per battery.