Frank, menus often include terms that are obscure - regardless of the language or country. Some is technical language use, some is over-elaborate description and some just unfamiliar when initially encountered.  It is worth asking your questions though - I am sure I would not be the only one here with significant interest in cooking (and eating) who would be happy to help. Throw them at us !

With regards to identifying the 'cost' here, I understand your confusion. The first paragraph is difficult to follow without context, I think. It seems to me that it is directed only at diners who have a 'bon cadeaux' - gift voucher - that has been pre-purchased, usually for them although it is possible to buy in advance for oneself if desired. The paragraph is explaining that the menu has 'evolved'/changed (become more expensive is the sub-text to me) and that pre-purchased gift vouchers only cover the cost of one of the 2-course options for dining, either entrée-plat ou plat-dessert, and that if the full 3-course option is desired, an extra (discounted) charge of 10 euros is incurred. The prices listed for each of the 2-course options and the 3-course complète are for those without a 'bon cadeaux'. And then of course, voucher or not, there are the side dishes, and drinks, which are not part of the 'menu à prix fixe' ! You probably know that it is not  uncommon in France to offer a 'prix fixe' in this manner, sometimes still offering a full selection from the à la carte menu, with supplementary charges for specific dishes.

Interestingly my French sister-in-law bought her son and his partner 'des bons cadeaux' for this restaurant last year, and it was much discussed around the table when we visited (they live in the commune of Marcenais, near Bordeaux), in advance of their visit to the restaurant. They subsequently thoroughly enjoyed the experience of dining there.