JavaScript (Node.js), 91 - 50% = 46 bytes

n=BigInt(process.argv[2])
f=x=>(x/10n|0n)-2n*(x%10n)
while(n>70n){n=f(n)}
console.log(n+'')

Ran the code in VS Code with command line input, and tested all the provided test cases. Added support for long integers, so claiming 50% bonus.