Hello,
If you just want to receive 433mhz signals, and from a letterbox I supose that’s the case, your best option it is rtl_433 : GitHub - merbanan/rtl_433: Program to decode radio transmissions from devices on the ISM bands (and other frequencies)
There is also addon for HA: Home Assistant Add-on: rtl_433 with MQTT auto discovery
You just need a DVB-T usb dongle