While many campsites will allow you to pay the tax on the day you leave, others – such as for a service taken from a travel agency or for a train ticket - have a policy of collecting the cost of the stay on arrival, or the balance if you have paid on account. You may not be happy about it, but it is not illegal.
Also, if you intend to use holiday vouchers to pay, find out beforehand whether the campsite accepts them, as not all professional tourist service providers take the necessary steps to receive the necessary approval from the National Chèques Vacances Agency.
Please note, too, that the tourist tax is never included in the price for your stay. The tax aims to finance the local authority’s facilities and public services, and is raised by a large number of municipalities in tourist areas. It must be paid onsite for an amount varying between €0.15 to €1 per day per person.