i buy 2 simcards. Why I get bills with double bigger price than they promsed?

I expected i have one number for family ans another for business. So about 7 euro for 2 numbers per month. One simcard is with 100 Mb for 2euro per month. 1st month i pay 4euro-its double more, 2nd month i pay 2euro.I wouldnt have any questons about that but 1st facture i get was more than for 13euro. I went to Post and they say its for internet i use exrta. But when read details can see that my 100Mb is not over!!! So why extra? But I pay this bill and let mobile at home didnt use it whole month. and next facture comes for almost 25 euro(!) And same story- overuse internet. About 400Mb extra but my 100Mb not used at all!!!
Another number i have with 1GB internet and there all ok but I have to pay 10euro instead of 5euro per month. I was again at Poste and they say its their mistake and they will call me during 2 days. NOBODY called. I still dont use my 1st number and for 2 months i have paid almost 40 euro for nothing. Why Post cheating people? They dont need clients? I want things be fair, Useless to go Post office again. Im really angry about the servive. I can prove all my words with documents and photos. I lose money, time and nerves.