Build and configure the system to fit your needs!
Combos are somewhat similar to addons: in both cases, you offer something in addition to the main service. This feature also increases the cost of the reservation and makes your customer much more satisfied.
Optionally, you can specify minimum and maximum limits for each service. In the first case, your client will not be able to make a reservation if he/she does not select the minimum number of additional services. In the second case, the system will not allow adding more extra services than the amount you specified.