Forced to make payment of booking

This vendor quoted on a group booking, proceeded to invoice us separately and then chased us for payment. Once payment was received advised that the group was too small then advised revised pricing - whilst on the invoice stated that if paid in full no increase was due. On enquiry - was treated with hostility, as well as the attitude of if you don't like it cancel. a 30% fee is then the penalty. Lack of transparency as they where aware that certain people were not travelling as they had cancelled directly with the vendor who took our money knowing that this would transpire.