Variation in payment

Before I upgraded my router My payments were 3/1/2023 R273.13, Then R273.13 on14/2/2023, Then R212.04 on 3/3/2023. Why the variation in payments. I then upgraded, see quote*** dated 3/3/2023 to the value of R297.14. Then on the 3/4/2023 my statement reads R316.25. On 3/5/2023My statement reads R297.15 and yet again on the 3/6/2023 the statement reads R379.24. And then I am also informed because I have a vodamail email address I now also have to pay another R29.00 Why all this variation in payments, Can vodacom please email me the explanation. As a pensioner before I do anything I have to make sure i can budget for it and then vodacom sneaks in these surprises???????????????