What is this species of green frog found in store-bought flowers in France?
This looks very much like a tree frog of which there are many similar looking species. Without knowing where your flowers came from or more characteristics about the specimen, it'll be difficult to provide an accurate answer*. 

One possibility may be the European tree frog (Hyla arborea).

According to Wikipedia:


  Members of the H. arborea species complex are the only representatives of the widespread tree frog family (Hylidae) indigenous to mainland Europe.



which is supported by genetic work by Stöck et al. 20081


Description [source]:


Small (3.2-5 cm in length)
Slender body shape with long legs and smooth dorsal skin+
Usually green (but variable) with dark brown lateral stripe across eye caudally to groin and whitish ventral skin surface.
Gripping discs on toes


The range of this species also includes much of France:



 Image Source 

+ Although the description states this species has smooth dorsal skin (possibly like the OP's specimen -- I can't tell for sure), many of the pictures I've seen seem to suggest that the dorsal skin of this species is actually rather granular. For example, see here. 

*Feel free to update your post with additional info and perhaps a better response will be added.



 Citations: 

 1 Stöck, M., Dubey, S., Klütsch, C., Litvinchuk, S.N., Scheidt, U. and Perrin, N., 2008. Mitochondrial and nuclear phylogeny of circum-Mediterranean tree frogs from the Hyla arborea group. Molecular Phylogenetics and Evolution, 49(3), p.1019.