Well your order has just been shipped otherwise we could have installed the RF transmitter for you. If you’d soldered before, this would be a really easy modification as it’s just three pins to solder. Also, this is easy to expand — if you need to control more devices you can just add more sockets, and there will be no extra wiring or relay.
That being said, I understand some users are more familiar with relays, and they are after all more reliable. Regarding the 24V relay you linked to: it’s not clear to me the coil runs on AC voltage. The page says coil voltage is 24V, with no indication whether it’s AC or DC. All the AC terms are referring to the contact voltage, and not coil voltage.