Thank you for your interest in our RC switches. The relay you link to does not have the same form factor as the sugar cube relay on the RC switch with relay so it might be challenging to wire it. Instead, I recommend using our Pololu RC Switch with Small Low-Side MOSFET with your 20A relay.
This board requires a 2.5–5.5 V power source supplied to VCC, so you will need to step-down the 7.4V battery voltage. A 7805 would work, but you might consider using a more efficient step-down switching regulator.
Alternatively, you might consider our Pololu RC Switch with Medium Low-Side MOSFET as it has a on-board regulator that allows it to be powered between 2.5-16V, though the MOSFET would be slightly overkill for actuating the relay.
Also, you will probably want to add a flyback diode in parallel with your relay.