The service items are in the manual here:
https://hondafitjazz.com/manual3/index.html - listed under maintenance.
The aux belt doesn't show a replacement mileage/age, but I agree with you, wise to get it changed. I bought a Gates belt for mine from Ebay for £9. No way would I be spending >£10 on an aux belt!
When I took my old belt off last week, it looked like new at 128k, but got replaced anyway. The old will be kept for spares.
Replacement is not too difficult. The tensioner sadly doesn't lock, so renewal is a bit tougher as a result. I broke a spalsh guard trim clip in the process as is my usual way, but again, a cable tie made a suitable repair.
The process for renewal is in the manual if you fancy having a bash.
£99 to change? Hours labour and full price part probably about right. I would be going non franchise and pattern part if it were my car and needed a garage to replace. Should bring the cost down a long way.