hi
You can serach for AMC where you can find relavent threads.
For AMC you can create task list with external services and create the maintenance plan as per the freqencey of visit , based on confirmation you can do the service entry sheet and cofirm the servies , FI will process the payment
Other way is create framework work create the service procurment maintenane plan while will create service entery sheet , and FI will proceed the payment
kindly check with the client how exactly do they want the AMC in SAP
Regards
Thyagarajan