Deducting more money than they should on a prepaid account

Our business makes use of courier services on a weekly basis. We signed up for The Courier Guy's prepaid service thinking it would be easier to just have them collect parcels at our office rather than us having to go to their depot to send parcels to our clients.

We would book the courier and select the economic overnight delivery option on their online portal. The correct amount would then be deducted from our account and the courier would pop in to collect. All good.

But twice now in a period of 6 weeks we noticed that afterwards even more money was deducted for the same parcel. The first time it amounted to R240 and the second time it was R178. The parcel's weight was conveniently changed to 3kg - for A4 documents! I'm seriously thinking that they're committing *****.

How many clients do not check their accounts and do not even know The Courier Guy is taking more money from their accounts after the initial booking?

After numerous emails they did eventually fix the first 'mistake', but now we're still waiting for them to fix the second. I want our money to then be refunded to our bank account (I did send them our banking details) and then I'm closing this account.