You will find other threads on here, this is a pretty well grilled topic.
Yes, reed switch or a hall sensor is the go. They are very cheap, small and light. Hence freight is so cheap it's irrelevant.
Em3ev also sell similar plug and play cut offs, no, they are not cheap.
The cheapest solution is what I will be testing when I get mine back = some of the brake levers that are supplied have a sensor/magnet arrangement built into them. I have a set in this state. I have disassembled it and will glue a magnet on one side, and the hall sensor to the other.
the main problem is not what to use to sense it, its having the right plug fitting on the other end is where it gets expensive. That's the advantage of dissembling a cheapo (USD3) set of brake levers to get the hall sensor/magnet + spares ftw. For the record, I didn't buy them specifically - I already had them.