I think that the problem may have arisen because you bought stamps in 'tabac's ...

Firstly most tabacs no longer sell stamps - they don't get any 'commission' - and if they do sell stamps to complement their postcards (because most people want to write and send their cards immediately!) they usually only have a limited supply and in limited denominations.

Whilst I appreciate that one cannot always plan to be somewhere specific at a given time ... 08H30-12H00 and either 13H30-17H00 or 14H-17H30 Monday to Friday and 08H30-12H00 on Saturday mornings, LA POSTE is the place to go and get stamps!

The pink stamps that have no printed value on them are 60 cents each and are intended for internal post up to 20 g in weight. The blue stamps that have no printed value on them are valid for sending letters and cards from France to member states of the EU and Switzerland to a weight of 20 g. They cost 77 cents each and for cards to the rest of the world up to 20 g in weight you will need a stamp costing 89 cents.

The French often like to send their cards in envelopes but that does not affect the tarif, unless it pushes the weight into the next band up, hence a pricier stamp.

You can buy 'carnet's (books) of pink and blue stamps ... they keep their value even if the price goes up so if you have any left over you can use them next time you're in France.

thanks for the clarification ALsa! Didn't know that the situation had changed.
It's a pity none of the tabac employees directed us to go to La Poste as the problem would have been easily solved and as we were mostly in tiny villages we didn't see very many.
Will be better informed on our next visit
Forgot to say we eventually bought the red ones & all cards arrived

Really glad your cards arrived with pink stamps on! Sometimes La Poste get it right, which happened to you. Sadly they also aim for black holes somewhere in the channel with remarkable accuracy for me these days, especially the type where you get notified that delivery was successfully made!!!