Do I have to spend the whole eCertificate amount?

You do not have to spend the full value of your eCertificate but this is not advised as your overall package price and monthly Hire Agreement costs will not be reduced if you underspend. 

Cyclescheme recommends that all participants redeem the full value of their eCertificates at the point of redemption, as it's essentially a 'use it or lose it' scenario.