Groups and schools

I need an invoice for my billing. How do I get one? Can I cancel my group visit? Do I have to pay for the tickets booked for my group in cash? How do I pay for the tickets booked for my group? My group size has changed. Do I need to report the changes? I registered my group online but haven't received confirmation yet. Is there parking for buses? Do we need to reserve our group visit or school trip in advance? Are there preferential rates for groups and schools?