Incorrect Income review by Momentum

I sent in my deceleration of income to Momentum in December 2018. Firstly they respond in the last week of December stating that they have reviewed my income. They have put me in a higher plan than what I was meant to be in. Apparently calculating income for the last 3 months determines the category of which they will be debiting monies from my account. They do not look at what your basic salary is. I earn a basic salary (of which is guarenteed to come into my account every month) and then if the company makes target, I then earn commission (which is not guarenteed every month)

For the last 3 months, I have been earning a nice sum of money due to commission which reflects in my bank account.

I called a consultant who argued with me over the phone about my point. I asked her, "what if for the next 6 months I do not earn commission" she said "But there is no guarantee that you wont earn commission"

I am the type of person does not like to over budget, just like most South Africans, I have rent and other expenses that I need to pay every month of which i have always budgeted on my basic salary of which is always guaranteed.

The conversation with the consultant was going nowhere and i ended up ending the call as it was clear the consultants and Momentum believe that all South Africans needs to over budget.

According to the bracket, set out by Momentum, I need to pay R1062 however due to the last 3 months being successful and earning a nice sum of Commission( i would like to express that this is not guaranteed) i have been charged R1479. This is a lot of money and as a part time student, I simply cannot afford this as of yet.