Thanks for the suggestion zjones4, I agree that buttercream might also be very important to this sinking issue. I have been using an all butter ABC because of the flavour (insist on that) and adding just a tiny bit of milk/cream (so it is a thick filling) but I know I will have to eventualy start using a part of shortening ( grr ) for stability.
I made some research and found that if adding a bit of gum tragacanth or xanthan gum to my buttercream (both flavourless) they will thicken it and make it more stable, has anyone tried this?
I will also try IMB (italian meringue buttercream) and see how stable it is under hot conditions, has anyone had successful experience using IMB (without shortening added) during the Summer ?
Usually I end up finishing some decorations on the cake before delivery so sometimes it is not transported as cold as desired, will definitely change that procedure...
And I think I should also have to make my buttercream layer of buttercream under the fondant a bit thinner. Is 1/2 inch a good thickness of buttercream under my fondant ?