Courier Guy ****med me

I sent a parcel with them, they charged me R428, paid then the following I received an email that the account is in arrears, logged in to check and it was an additional R151, decided I will pay it the following day and the following day when I logged in, the arrears had changed from R151 to R6545.80, so now they are refusing to let me take my car seat and use another courier coz I am not about to pay R7000 to transport a car seat from burgesfort to joburg