Symptoms
Several service plans are chosen in the on-line store (e.g. several domains are attempted to be registered), but as a result there is only one (the last one) in the shopping cart.
Cause
The service plans attempted to be purchases are included into the same unique group: Home > System > Settings > Unique Groups which includes mutually exclusive service plans.
Resolution
If you need ability to purchase several plans it is required to delete this unique group and synchronize the store.