I'd encourage you to have a stab at working this out yourself first to see what the potential value to the company is.
This thing I've created turns a 30 minute (and longer) job into a 10 second job
Maybe estimate how many times a day each depot does this 30 minute job. If it's say 10 times, then that's five hours labour per depot per day = 30 hours a week = 1500 hours a year per depot.
1500 x 20 depots = 30,000 hours x £15 per hour labour? = £450,000 per year (+£100k software company saving) = over half a million quid saving.
Ask for the £100k they would have paid the software company as a bonus