unreasonable Invoice

I got demo site for testing code only, and i did activated "Cache Reserve Writes" just to test the service, i may need it later for production. and that did cost me $4.50 for 47 Writes. I was expecting it's pay-as-you-go not per 1 Million. Invoice number IN20709971 i don't believe this is fair pricing model, and i don't think it's fair to pay $4.50 for 47 or 10K writes.