Mix telematics going back on their promise

I am an existing client of Mix Telematics. I was told by the sales lady at the call centre that they would send a technician to me to have a new unit fitted in a new vehicle I want to add to my account, it's a TLB (Tractor Loader Backhoe machine) that cannot be taken to a fitment centre. Mix telematics now tells me that I will need to pay half of the callout fee for installation (which almost adds up to my 1st 3 months subs), so they have gone back on their word. I offered to collect a unit and have my auto electrician install the unit for me but they refused.

I don't see why I should pay for the installation when I was told they would do it for free.