Who the Hell is Vodacom to dictate how I spend MY money

I am on pre-paid with Vodacom. On Tuesday I had an airtime balance of R41. I bought recharge vouchers to the value of R440 and recharged my airtime, leaving me with an airtime balance of R481. This was to buy a 5G data bundle for my Wi-Fi router. When I tried to buy the bundle, I got the message that I had insufficient airtime to make this purchase. After speaking to numerous people in the call centre (all giving different advice - switching off my phone over and over again, putting SIM into another phone also over an over again), I got to speak to Judy Kubheka (very rude and talking over me the whole time that I eventually had to tell her to shut up so that I could speak) who told me there is a 80% rule, i.e. I can only purchase a data bundle for no more than 80% of my airtime balance. Now my question is - Who the hell does Vodacom think it is to dictate to me how I should spend MY money. Vodacom did not give me the airtime, they did to gift it to me, they did not advance it to me, I BOUGHT IT WITH MY OWN MONEY so surely I have the right to spend as much of it in any way I chose. When I go into a grocery store, they don't ask me how much money I have and restrict me to only spending 80% of what I have, so who the hell does Vodacom think they are?????????? The only way I could get this resolved was to go out in MY CAR with MY PETROL and buy another R29 recharge voucher to overcome this Vodacom imposed 80% rule on MY MONEY. And, I had spent over 4 hours on the phone with various Vodacom consultants, etc. to try and get the problem resolved, but to no avail!!!!!!!! I am a very disgruntled customer!!!!!!!