Contract manager struggles with client billing errors. How IT help desk software can simplify manual contracts???

Hey all, our service contracts billing is all manual right now. Someone has to go through each contract, check hours worked, match against terms, calculate amounts, then enter into the system. Keeps generating errors like wrong rates or missed line items and clients are getting pissed.

Been dealing with chargebacks and disputes every month. Need something accurate but not sure where to start. What tools or systems do you use for contract billing or any on prem options or simple ways to automate this without big costs??

What works for you guys?

Thanks!